JosephScha: well seen and captured
aardvark7: I am sure that someone will be able to explain why we can't have our cake and eat it...
My immediate reaction is why wouldn't the lens manufacturers offer this, along with their existing line up? I presume there is a good reason.
you can use a Pentax PK to EOS adapter and buy the canon version. Almost all brands can easily be converted to canon EF mount.