All modern Pentax SLR bodies can mount all K-mount lenses, but there are some problems. I don't shoot Pentax any more, but I know that the new bodies can't control the auto-aperture mechanism used on -FA and earlier lenses, and new bodies can't get proper exposure readings for stop-down aperture settings, without using a special mode provided by a green button.
You should visit Bojidar Dimitrov's Pentax K-mount site: http://www.bdimitrov.de/kmp/, it's the ultimate equipment resource for Pentax shooters.
As jcolwell said... ALL K mount lenses can be used on any Pentax DSLR. Obviously the manual focus ones will not auto focus but they work perfectly well in manual. As for setting the aperture, the "green" button will sense the set aperture and give you the averaged meter reading. This step is obviously awkward in fast changing light and makes full program mode difficult, but it is no different than setting your exposure manually on vintage cameras. Of course, this is something many of us do as a matter of course because why should you use a $1000 DSLR like a $200 P&S.
