Hey Simon,
(I replied
over at mff as well, check it out if you haven't yet.)
The only normal adapters for Pentax are for M42 (which work in stop-down but otherwise as intended) and for medium format, like Pentax 645. There is not enough distance to house the bayonet of a Nikon lens or of a Leica R lens, I have tried to fit both.
You can convert Contax/Yashica mounts to Pentax K mount with surgery that isn't too bad.
Other than that, you're restricted to Pentax K mount lenses. And as many of us here have discovered, if you use a pre-A lens (K and M series, or even M42 lenses) the newer bodies like the K10D have exposure creep as you stop down. There is no such problem with the *ist D or *ist DS, and replacing the focus screen in the K10D with one from the DS solved my exposure woes.
Good luck!
Sean