Good timing for new camera owners. Apple's Digital Camera RAW Compatibility Update 2.6 brings Raw support to the new Canon T1i (500D), Canon EOS Kiss Digital X3, Nikon D5000, and the Olympus E-30. Since the update is implemented system-wide, the upgrade applies to Aperture, iPhoto, and Preview applications.
I had Canon T1i and Olympus E-30 Raw files already in my Aperture library. This meant that I could look at thumbnails, but if I tried to view anything larger, I would get a "file not supported" message with no image. After applying the 2.6 update, the message went away, but Aperture still had a hard time generating full size previews from the Canon T1i Raw files.
When I uploaded new T1i Raw files to Aperture after the update, however, everything worked great, and the new Raw decoding for the camera was quite attractive.
So I guess the moral of the story is not to add Raw files to Aperture until the camera is officially supported.