Correction - M42 lenses will work with a proper adapter (with no flange, like the one in this picture >
http://www.mattdentonphoto.com/images/pentax_k_m42_adapter.jpg )
They don't work when using other kinds of adapters, because the contact pins are shorted.
And they DO accept "A" lenses. Basically any K-mount lens with an "A" position on the aperture ring (or no aperture ring at all - FA J, or third party)