I planned to write a brief response, advocating outreach to the environmental community (and to the green blogosphere, which is extraordinarily active and vibrant!), but as I chewed on it, I was inspired to go a little further.  

"Expanding our base" is not the only reason the netroots should be reaching out to environmentalists, and making environmentalism a legislative priority on par with net neutrality.  I can offer, off the top of my head, TEN REASONS.

1) Multi-issue legislative outreach.  The environment is a topic that tangentially interfaces with many other progressive issues.  The environment is about politics and economics.  It's about social justice (feminist, Latino, African-American) issues.  It's about morality (and yes, religion).  And you better believe that it ought to be about good old-fashioned self-preservationist survival!  
I would go so far as to say that environmental sustainability could become a guiding principle of progressivism, along the lines of Chris Bowers' ongoing consideration of the principle of pluralism.  It has this strength as a foundational value precisely because of its multidisciplinary, multi-issue nature.  It is the kind of vision that could form the backbone of a strong, flexible, dynamic party platform.  

2) Ignoring the environment is self-deceptive, if not impossible.  Along related lines to #1: I've been writing for a long time about the mistake (made both by self-identified environmentalists and by others) of conceiving of these concerns as "special interests."  Environmental concerns are EVERYONE'S concerns, and at its core, the environment is, quite simply, EVERYTHING.  Our surroundings, our passions, and our own bodies will always and forever be a part of it.  We cannot, and indeed, should not, seek to escape it.  It is ours- not in terms of ownership, but in terms of identity.  

3) "Megaphone" effect.  Working with environmentalists more directly offers a way to use already-present progressive infrastructure to expand the influence of the netroots (organizations such as the Sierra Club have been around for a century).  A legislative environmental victory would bring praise and attention to the netroots, and could serve as the foundation for a stronger long-term alliance.  On that note-

4) Long-term coalition building.  The netroots is a diverse group of progressives that have come together in recognition of the power of partnership and community, as a means of achieving long-term goals.  In 2005, significant controversy erupted over a provocative paper proclaiming "The Death of Environmentalism".   Authors Shellenberger and Nordhaus called for the "death" of the current environmental movement, so that another (more effective) one might "rise from the ashes."  One of the ways in which the current movement is cited as ineffectual was that it has built "tactical, not strategic" alliances with other progressives- that these have been temporary at best.  Many environmentalists have since arrived at similar conclusions, deciding they won't simply "work with" unions, or feminists, or government reformers (or Democrats!) when it's convenient, but must instead be in it for the long haul.  This community is ready for our message of partnership.  

5) Keeping environmentalists away from that Nasty Green Temptation.  Environmental policy is my top priority, and this can be said of other progressives, as well.  In light of that fact, there's a danger that people like me might, oh , I dunno, vote for the Green Party in key elections (gee, can anybody remember a time that happened?), thereby "spoiling" it for the Democrats... netroots Dems included.  The only way the netroots can legitimately mitigate this problem (besides whining about it in threads, and calling environmentalists names, which I've witnessed far too often) is to proactively draw environmentalists into our community, and create a principled unity so compelling that no one will be tempted by that dangerous, sparkly Green glow.  

6) Keeping environmentalists away from the Republicans.  As I indicated above, the environment can be something of a single-issue deal-breaker for me, one of the only things that might tempt me to buck party loyalty.  For instance, if Phil Angelides or Sheldon Whitehouse were not environment-friendly themselves, I might have had trouble ignoring the Governator, or soon-to-be ex-Senator Chafee (luckily I wasn't ever in that position - both Dems mentioned above are solid on environmental issues).  
It's clearly in the netroots' interests to keep environmentalists away from the Green Party, but this other danger is real too.  Conservatives everywhere are beginning to realize that the environment isn't an issue they can be on the wrong side of much longer.  Exhibit A is the Tory party leadership in the UK, and if you'll look over here, Exhibit B is the breaking news of a carbon tax proposal being discussed, of all places, in GOP policy circles!  They just lost bad, and they're hungry to try something new.
Bad news for the Dems.  In the long run, it is in environmentalists' interests (in accordance with point #1) to avoid one-party loyalty, because environmentalists desperately want both parties to embrace what basically DOES amount to a no-brainer forward-thinking agenda.  This is a danger the Dems would do well to address in advance (for myself personally, of course, if both parties were strong on the environment, I'd vote Dem for all the other reasons there are to do so).

7) Vital component of the Western Strategy.  I wrote on this extensively here, so I won't go in to detail.  But in brief, environmental values are becoming an important part of the New West, and candidates from Schweitzer and Tester to Richardson and both Udalls have embraced them in tandem with traditional Western values of independence and outdoorsmanship.   I also believe this will help avoid the danger of any harmful "Sagebrush" tendencies creeping in to Kos' meme of the Libertarian Democrat.

8) Our finest netroots candidates can't be wrong.  Jon Tester.  Jerry McNerney.  John Hall.  These are people who have spoken passionately, emphatically about protecting our natural heritage, moving toward energy independence, and safeguarding our children's right to clean air and water.  We helped elect 'em, so let's trust 'em!  They get it.  

9) Win-win scenarios abound.  It's pretty tough to attack "supporting the environment" - which is why everyone says they do, including our president.  In fact, in some surveys, more than 70% Americans self-identify as environmentalists, even if they don't always rank the environment as high on their priority lists as I might hope.  Environmentalists have lost so many battles largely because of a failure to engage in effective framing.  
For a superb, in-depth analysis of the last few decades of environmental political heartbreak, read this, and you'll see environmental opponents' success at framing the debate repeatedly.  

10) It's the right thing to do.  Last but not least, of course.  C'mon, folks - many of you have seen An Inconvenient Truth, or noticed how many kids these days have asthma, or read somewhere about the tens of thousands of species disappearing under our noses.  You've got too many friends with cancer.  Or you know about the suffering of millions of people in arid regions of the globe, where water shortages are resulting in refugee populations growing fast enough to collapse whole governments.  We pay at the pumps, and then again for our war in Iraq, driven in part by a desperately insecure geopolitical struggle for oil.  

And I dare any of you to tell me you haven't once stood, alone, and felt the thrilling, awe-inspiring immensity of nature wash over you.  The environment - it's not just the only reason we're alive - it's the greatest thing we are.