The Canon people have been using adapters to mount k mount and m42 lenses on their camera bodies for a while. They even have adapters to use Pentax lenses on EOS EF mounts. What is actually trickier is to adapt the FD mounts to EOS. Its ironic that it is easier to adapt m42 and k to either FD or EOS than FD to EOS. For example, I have a special canon adapter to mount a FD Macro to my EOS 10s. It only works with correctly with the macro. There are some Canon adapters for a few other FD lenses. The distance to the focal plane is tricky on the FD to EOS and require a optic which causes a slight magnification to get the lens back to infinity focus. Canon didn't make many adapters so they are scarce. However there are some aftermarket companies that make all kinds of adapters. Cirrus is one out of Hong Kong.
What irks me is the canon guys will often modify the k mount lever so it doesn't cause problems on there body. Some of them remove it completely. I have seen this with some nice Pentax primes.

On a separate note, I have a adaptall 2 EOS-M mount that is relatively rare and I got it by accident. I was over at MF and saw a thread where someone in England was telling a guy that Tamron never made and adapter for EOS, that EOS came out after the Adaptall 2 line was discontinued.

I registered on that site just to set the record straight. I need to take some pics of the Canon adapter and Tamron I have so people will believe it. The only reason I haven't gotten rid of the canon body and 2 lenses and adapters that I have is that a can macro flash is the only macro flash that I have. I have that about getting a digi rebel to use this stuff on but I end up having convulsions.
PKA to Canon EOS EF mount:
http://www.amazon.com/Pentax-Canon-m.../dp/B000YDC1W8
M42 to Canon FD
