When Obama appeared on Letterman as the show's only guest, he took a page from the George Bush playbook and bypassed the media to deliver his messages unfiltered and without pesky criticism, sort of like when Dick Chenney appears on Fox.
You could argue that he is diluting his premium brand by appearing on so many talks shows after he also blanketed the Sunday morning talking head panels, but he rightly thinks passing healthcare will define the next three years of his presidency. (Well, healthcare and the economy stupid.)
He also decided to take on Fox News. Figuring he has little to gain by playing nice, Obama declared war on the network, arguing the network's opinion makers rarely side with him on the issues of the day. In the Obama calculation, it was safer to call Fox News the opposition party megaphone rather a vendor of fair and balanced news. Shooting the messenger never gets old in Washington and the administration decided Fox's audience wasn't going to vote democratic in the next election anyway.
The next election cycle will tell us if Obama should have used a bigger carrot instead of the stick when dealing with Fox News.