you lose functionality though (on the olympus; and i believe the situation is similar with old nikon lenses on nikon DSLRs) as there are only electrical couplings on the DSLR lenses, and only mechanical couplings on the OM lenses. e.g. whatever aperture you select on the lens is automatically engaged, not just at exposure time. some metering modes don't work due to lack of said coupling, but enough do that that's not a huge issue. also, olympus don't recommend all apertures - given the size of the CCD sensor (smaller than on nikons or canons), the focal length conversion factor is 2x, rather than the more usual 1.6x.
i think with the pentax once you use a custom function to engage the appature ring you can either use it as av-priority or metered/manual. Not 100% sure on this, but needless to say will check it out if I get serious about buying it