I have no idea, but judging by the teaser trailer that is playing in front of Inglourious Basterds, the movie is going to be some kind of awesome.
Update: In Contention's Kris Tapley has scored a fairly detailed description of the plot of Inception. Go here to check it out, but be warned there are spoilers galore. Plus it is so complicated, it made my head hurt a little. I stopped reading after the first few paragraphs.
The only nugget Inception's page on imdb offers is "A CEO-type becomes involved in a blackmailing scandal." Apparently, said scandal involves zero-gravity fistfights in hallways and lots of scenes in which Leonardo DiCaprio looks totally freaked out.
Like Following, Memento and The Prestige, Nolan wrote Inception himself, which essentially translates into "There Will Be Head Games." I really like the ominous, moody music on the trailer, too.
Whatever Inception turns out to be, you know Warner Bros. is high on it, because they're advertising the movie nearly a year before its release date of July 16, 2010. Curiosity officially piqued.
Somewhere deep within the editing bay of Avatar, James Cameron is fuming with envy.