The Illinois Legislature requires a constitutional majority (51% of ALL members) to vote for something to pass. This is different from most state legislatures of the US Senate where a simple majority (majority of anyone who votes that day) is necessary for a passage.
What this means is that voting "present" will shoot down a bill/proposal/whatever in the Illinois Legislature as surely as voting "no". Let me repeat that: IT WORKS THE SAME WAY AS IF HE VOTED NO.
And before you start showing how little you know about this subject by going on about how Obama was playing a CYA game let me tell you two more things: 1.) Illinois is not a blue state beyond the Chicagoland areas and a few college towns. It's as red as Indiana or Kentucky. 2.) After his present vote that shot down a bill to use extraordinary measures to save an aborted fetus if it survived the abortion, Obama went out and spoke to the press and took shit for it.
(I know this because I've lived here my entire life and because I actually investigated the issue.)