According to Wikipedia (okay, okay -- maybe not the most reliable source) [http://en.wikipedia.org/wiki/Landslide_victory], a landslide win in the U.S. would be 60% of the electoral votes, or 55.5% of the popular vote.

I haven't seen any numbers yet on the popular count, but if 60% of 538 total votes in the electoral college constitutes a landslide victory, Obama's 338 (so far) has already beat the minimum of 323 by 15 votes.

Missouri (11) and North Carolina (15) have yet to be weighed in, and it would be excellent if he gained one or both of those.