Al's post seconds what I have been thinking as well. Obama is smart enough and has sufficient legal training to realize that immediately challenging the bounds of Heller with the existing slate of Justices is going to be a further loser for the anti-Second Amendment crowd and it will hurt him politically as well.
I suspect they will follow a two-part strategy.
1) Nominate judges who will undermine Heller in the lower courts and block the better formed cases from reaching the Supreme Court
2) Replace that 5th pro-Heller Justice on the Supreme Court
Both of those strategies can be done without angering gun owners or costing him and his party politically. When he has sufficient ground there, all he needs to do is wait for the inevitable shooting that catches some media buzz and use it as a platform for more gun control.
However, I generally agree with buzz_knox that the Democrats supposed move away from gun control has nothing to do with a change in intentions; but is more about them realizing the wisdom in cloaking their intentions. Remember that this strategy was masterminded by Chuck Schumer in 2006 and Chucky is not now, nor has he ever been, a friend of gun owners.