When I got my K10D, with the 18-55mm kit lens, I tried the lens on my SF1 film SLR, just for fun. Of course, it functioned perfectly, aperture and autofocus worked just as they do on the K10D.
There was vignetting at focal lengths below about 35mm. Above that, the image filled the frame. There was a little bit of barrel distortion, which disappeared at the longest focal lengths.
So, yes, the kit lens will work on a FF camera, but it really isn't practical. The same is true for all DA* lenses. Some will fill a 24x36mm frame; some will not. They are designed for the APS-C frame, so there are no guarantees. Edge sharpness may be lousy, as well, since the desingers didn't care about anything larger than 16x24mm.
Of course, its a moot point. IMHO, Pentax will never release a FF dslr. FF is a niche market that is dominated by N & C, witha little competition from S. Even if Pentax could compete in that market (which is questionable), there just isn't all that much money to be made. If S, with their much-deeper-than-Pentax' pockets, can't make a dent in that market, what hope does Pentax have? Its not a rapidly growing market segment.
IMHO, Pentax would be better off spending their money trying to gain a foothold in the MILC market, which is still wide open, much larger than the FF niche and is a much larger potential market. In such a market, Pentax stands a much better chance of being profitable than in the FF market. I think that Pentax realizes this, as well. Witness the Q and the K-01. Both are attempts to create a MILC that appeals to the mass market, not the rarified atmosphere of the FF market.
In general, its not a good idea to go after two market leaders in their stronghold. A better strategy is usually to "hit 'em where they ain't". That's the MILC market.
But then, what do I know?