According to DemConWatch's numbers, Obama has 1712.5 delegates right now (if Obama gets Michigan's 55 uncommitted delegates and the 67 he won in Florida's primary)  That's gotta be Hillary's best case scenario (unless she really thinks that Obama won't get any of Michigan's uncommitted delegates).

Thus, if Obama's predictions stand he will have 1776.5 by May 20.  (I do think he's a bit too optimistic in KY and WV, though).  That leaves him just 7.5 delegates short of your standard of 1784 pledged delegates.  So in Hillary's best-case scenario (and accepting Obama's predictions), Obama will clinch the nomination either on May 31 (if the Committee accepts any solution other than Hillary's best case) or on June 1 when Obama will surely win the 7.5 delegates he needs to get to 1784.

I think the best you can argue is that Obama will call the race too early by 1.5 weeks because he's short 7.5 delegates.  That's a little nit-picky, dontcha think?