Well, it's true. Many of the lenses that work well, or workable on my 1Ds, 1D II, 30D, 20D, etc, would not work on the 5D. The butt of the lens would hit the 5D's reflex mirror. This includes Carl Zeiss the 45mm f2.8 Tessar pancake, Carl Zeiss 50mm f1.4 Planar, Carl Zeiss 60mm f2.8 SP Makro, Carl Zeiss Rollei 50mm f1.4 (hit mirror on 1Ds but still can take pictures). I will have to check the others, but so far it's not looking good.
Basically there are two solutions to this: shave the mirror by about 2mm, or shave the butt of the lens. I am not going to modify my 5D, and may file the rear of the lenses. I will have to assess if it's worth the trouble.
On the other hand, I am enjoying the 5D tremendously. It's one heck of a camera and the used ones are at bargain prices right now. If you always wanted a full frame camera, but don't want to pay through the nose, 5D is your camera. Some would complain about the built quality of the 5D, but I don't know what the fuss is all about. Other than not weather sealed, the 5D is well built. I have use both the 1D II and 1Ds for a while, and holding the 5D in my hand does not make me feel the 5D build is inferior. So, I really question those who complain about the build-quality of the 5D, what their motive is.
Carl Zeiss Tessar 45mm f2.8 pancake & 5D -- Notice the bottom quarter of the picture is black. The sensor was partly blocked by the mirror when the lens is at infinity. The lens itself, is extremely sharp
Men at Work -- 35mm f1.4L & 5D. On the other hand, AF lenses work wonderfully on the 5D without focus issues