Will old lenses for a film SLR camera work with a new digital SLR camera?

My mom has an old Cannon A-1 and an old Cannon EOS film camera, both of these cameras are from the 80s or early 90s and I was wondering if her lenses would work if I bought a new Cannon digital SLR camera such as the Cannon Digital Rebel XT or a Cannon EOS 20D. Or better yet if I got a new D-SLR would these cannon lenses work with any kind of SLR say for example a Sony or a Nikon?

The entire Canon A series, including the A-1 from 1978, used the FD lens mount system. That mount is incompatible with the EF mount (introduced in 1987) that is used on all Canon digital SLR cameras. So the lenses for the A-1 will NOT fit on a modern Canon or any other brand. You can get an adaptor ring for those lenses, but even then they'll have major problems.

The EOS camera uses the EF lens mount. So the lenses you have for that camera WILL work on a Canon digital SLR - and only on Canons.

Regarding the digital bodies, the Canon 20D has already been replaced by the 30D (essentially the same camera with minor upgrades), and there's a rumor going on in Canon user groups that Canon will announce a replacement for the Rebel XT any day. None of these cameras has a split prism focussing screen however, so focussing with old manual focus lenses can be quite tricky!

The Rebel XT and the 30D are both excellent cameras and good value for money, but if you want to compare brands look at the Sony A100 and the Nikon D80 (available as of next month). These 10 megapixel, $1000 cameras sit squarely between the Rebel XT and the 30D. And then there's the Nikon D70s, which has dropped in price to match the Rebel XT at $700 (making the Nikon the better deal, imho). And finally, at the budget end, there's the Nikon D50 for just $550 (without lenses or a memory card).