(This began as a comment I wrote at Tam's)
Here's something I've not seen mentioned yet.
Obama just publicly identified who he wants the Republicans to nominate to run against him.
Remember how McCain was the media's darling prior to the '08 Republican nomination convention? Let's see if the same tactic gets recycled. Push The Donald as the "obvious" "best" "choice" for the Republicans, then savage him afterwards to Obama's comparative advantage.
By declaring his intention to run for re-election as early as he has, Obama pre-empts any serious challenge from within his own party (something that was beginning to get MSM notice only a few weeks ago). Now he's demonstrated the "effectiveness" of his potential opponent by apparently being bested by him on a long-standing issue.
Palin let herself be marginalized in '08; who's going to fall on his/her sword to give Trump some apparent political expertise in '12?
As strategy go, Obama offers an interesting example. Deliberately make yourself a figure of mystery and target of extremist interest so as to control much of your opposition's efforts to defeat your active efforts.
UPDATE: Rush Limbaugh posits (about 42 minutes into the first hour) on his radio show today that the timing of this data release is more in response to a recent poll showing only 38% of voters think Obama "definately was born in the USA". I don't think the two theories are mutually exclusive. Trump is a very beatable political opponent (when compared to any of several potential alternatives within the Republican party), and scotching this non-citizen idea now offering more advantage than continuing the charade, are both viable considerations in Obama's re-election program. Either or both could have influenced the timing on this.