It's only been two months since David Letterman said goodbye to late night television, but he's already back at some of his old tricks, briefly coming out of retirement this weekend to roast Donald Trump.On Friday night, a bearded Letterman made an appearance at the San Antonio stop of Steve Martin and Martin Short's "A Very Stupid Conversation" tour, dusting off his "Top 10" gimmick to send-up the billionaire GOP presidential candidate with "Top Ten Interesting Facts About Donald Trump."