My sister had a themed wedding a few years back and it was great fun!
She gave each table a different film theme (ours was Shakespeare in Love). There was also Austin Powers, Grease, Amadeus to name a few. It was held in a Scottish castle so bride and groom were Scottish themed. I have to say the photos looked amazing (she's a wedding photographer herself so it was all about colour and style lol!)
For my OH and I the costumes were dead easy - we hired 16th Century style get-up and my dress hid a multitude of sins. Loved it!