I don't believe Trump is serious about running in 2012. Instead I think he is just selling his brand and using the GOP primaries to do so. The birth certificate issues simply helps Trump make a big splash. Since he is not really running he has nothing to fear about bring up Obama's birth certificate. So what if he gets labeled a birther, if he isn't running there simply is no damage.
I do think Trump has picked a good angle to raise the issue. Instead of saying he wants to see the birth certificate to prove Obama is American, Trump tries the tract of what is Obama hiding.
As Legal Insurrection notes, the applause proves Trump maybe on to something. After all, if you are a real American citizen, then why spend so much money and time not showing your birth certificate. Produce it and shut everyone up. Unless of course as Trump suggests there is something awkward on it that would not be so. easy to explain ( I always believed this to be the reason behind not producing it).
At any rate, expect Trump to beat this drum all they way up to the time he "drops out".
Via: Legal Insurrection