My quick thoughts:
On Obama's backing it:
I thought he might back this, albeit after he won a second term (if he does indeed win a second term, which I believe he will, Obama vs. Romney isn't enough of a show-stopper for the GOP to topple Obama, in my opinion...ironically, as much as I can't stand his views and aspects of the man himself, Ron Paul would probably be a more dynamic and "real" choice, Obama vs. Paul is much more a pro/con sort of evaluation of real differences, Obama/Romney, for lack of a better metaphor, is Coke/Vanilla Coke, and Incumbent Coke Classic wins that EVERY time!) ;)
On the issue:
It's about time, YES, gay marriage should be legalized nation-wide...I entirely disagree that this is a matter to be decided amongst the states, as 1. That's led to nothing but more of a mess over the issue (see: my native California saying "Yes" then "No" and now "Maybe" pending another ruling on Prop 8) and 2. SEPARATION OF CHURCH AND STATE...if they can find a place that will marry them, they should be allowed to without the state butting in and saying their love isn't "valid" enough to be considered marriage, with all the legal and social benefits that entails.
On the impact it may have on the 2012 race:
Well, Obama was never going to win in the South as it is...
The more North you go, the more swing-states it may play well in...but how well will this go over in those two big Electoral gamble-battleground states, Ohio and Florida?