Completely different is The Last Days of Disco by writer/director Whit Stillman, who also wrote and directed the cult classic Metropolitan. Both movies deal with the world of preppies and class and I love watching The Last Days of Disco for the outfits. It's funny that some of the most stylish girls starred in this film including Kate Beckinsale, Chloe Sevigny, and Tara Subkoff, the founder of the fashion line Imitation of Christ.
The last Gwyneth Paltrow movie I have to mention is The Royal Tenenbaums. She gets to wear fab furs and an Hermes Birkin bag but I love Luke Wilson's Bjorn Borg style tennis outfits the best. Quirky doesn't even begin to describe this movie but it's fun.
I know I've probably left off dozens of fashionable films but this is my list so let me know some of your favorites. It seems like Atonement and Miss Pettigrew Lives for a Day are already destined to be classic fashionable films of the future and quite possible Flawless as well, the movie that started this all in the first place.