Over at Slate is a great article — actually, it’s more a great conversation — by people with their own theories Inception. (Brow Beat : Five Ways of Looking at Inception)
While I like what the author, Nina Shen Rastogi, has put together as a list of ways to think about the movie, I highly recommend you also read through the comments. They are insightful, and the debate and discussion going on in that section is really good (unlike the commentary on alot of other sites).
Afterwards you may want to go back and give the film another look. I have seen it twice now and reading this article and the comments makes me want to go back and see it again.
P.S. There is talk about there being alot of material taken out of the movie: if so, that could make for a fantastic DVD when it comes out.
P.S.S. While the film is very intelligent in terms of content, it is a great film in cinematic terms, too. Here’s a trailer.
Smart People I Know 