Sivan Lewin told me that real architectural and fine arts photographers use Tilt/Shift lenses to avoid problems with converging lines and false perspective. These problems occur when you tilt the camera up towards the top of the building or scene to capture the entire subject. To avoid converging lines, the film plane (or back plane of the digital camera) should be parallel to the front of the building, and the camera should be positioned exactly halfway up the face of the building. This is great if you have a cherry-picker, but if you are standing on the ground, even a 6' tripod is still not going to get the height you need to avoid the distortion.
Enter the Tilt/Shift lens. It allows you to shift the lens up while keeping the back of the camera parallel to the scene, and thus avoiding the distortion (see this page for a comparison). I rented a Canon TS-E 24mm lens from Alkit Camera up by Union Square ($35/day). The lens fits my Digital Rebel, and still allows you to use your light metering. It is, however, manual focus. The lens performed very well on the long exposures I took- with very little light fall-off in the shifted shots. I will probably rent the lens a few more times before I purchase one myself. Bottom line: combined with a tall, sturdy tripod (like the Bogen DigiCompact that I bought today), a shift lens really does let you take better pictures.