I knew it! I knew he would drop the seal after his ridiculed by the blogosphere and MSM pundits:

Barack Obama’s campaign has canned its use of an emblem that critics had panned as a theft of the official U.S. presidential seal.
[…]
But another campaign source told The Atlantic.com that the campaign decided to stop using the logo because it didn’t want to give the press a reason to label Obama arrogant.

Actually that probably should read: “to give the press yet another reason to label Obama arrogant” 🙂
What’s really amazing about all this is that they didn’t realize this before they ripped off the presidential seal.
(via)