Yes, both do have M42 lens mount adapters. Nikon adapters must have an optical glass in the adapter to correct infinity focus. This will make most wide lenses, those 35mm and below, unusable due to their protruding rear ends. Then, some Nikon bodies won't meter with manual lenses. I'm not sure if Canon meters manual lenses though but I'm sure focusing to infinity won't be a problem.

If you still haven't bought a body yet, consider the Pentax dSLR. It can also use M42 mount (also called Pentax screw) lenses through the use of a cheap adaptor. Make sure the adaptor you get is flushed on the camera once installed to be able to focus to infinity correctly.

The best part is Pentax bodies meter with the old lenses. Pentax has openly claimed that all lenses they made will work with their newer digital bodies. I have a box full of M42 lenses from 28mm to 500mm under several brands, not only Pentax, and they all work with my K200d body perfectly. My dSLR also has a focus assist feature that helps me take sharp photos even with manual focus lenses. With aperture-priority, I have adapted to digital photography without changing my shooting style from film. This is the main reason I remained with Pentax when I decided to get a digital backup camera to my film SLRs.