Regardless of Clinton's motives for offering Obama the vice-presidency, it expressed a reality that Obama has yet to face: Neither candidate can win in November without the supporters of the other. Obama's response was not only childish and arrogant, it was stupid. Yes, he's ahead by a few votes. But to translate that tiny lead into: "I'm number one and I don't need anyone" makes him sound like a Republican. If he thinks he can win in November without Clinton's supporters he's already let his popularity blow his ego into Bush-bubble proportions.
All he needed to do to field Clinton's offer and still look mature was to make the counter-offer. Offer the vice-presidency to her, instead of puffing himself up and huffing about his little lead. The truth is, he's hardly any closer to winning than she is, and the longer they battle it out, the easier it will be for McCain in November.
Obama needs to remember that while the Republican party believes in a majoritarian kind of pseudo-democracy, in which winning 50.001% of the vote entitles you to act like a dictator and ignore the 49.999% of the populace that didn't vote for you, the Democratic party has always embraced a more genuine consensual model--trying to represent the nation. This is reflected in their different primary approaches--Republicans: winner-take-all; Democrats: proportional representation.
It's the height of hypocrisy for Obama to talk incessantly about 'change' and 'hope', while engaging in the same sort of macho, I'm-number-one-I'm-the-decider silliness that has brought our nation to its present state of decline.
We expect more of him.