If you edit the [SET gxResolution "YYYYxZZZ"] in your wow/WTF/config.wtf file, you can set the resolution to anything you want. I've never had it not work with any aspect ratio I've tried, and I've tried some pretty wacky ones.

Keyclone can do this with its maximizer as well, and in fact, I believe it does it with this same interface.