Leo Donofrio thinks only Quo Warranto action brought by the US AG or the DC Circuit Court could be successful now that Obama has been sworn in, absent impeachment. Quo Warranto, unlike a civil or criminal lawsuit, puts the burden of proof on Obama to show that he is eligible. See Leo’s site for details:
http://naturalborncitizen.wordpress.com/
Yeah, he’s been saying that since, at least, January.