Seriously, how can someone possibly review Children of Men in less than 160 chars???
The idea is to give a feel for what you feel about it and why. 160 characters is easily enough to do that. And you can convey a lot of emotion in the meantime:
http://www.blippr.com/movies/5182-Across-the-Universe/blips/...
If I'm looking to see if I'll like a movie or not, a review like that is perfect.