There is a simple reason that I'm not very excited about a 'FF' Pentax DSLR. Pentax is a small company, if it builds a FF camera, that means that in all likelihood there will be no top of the food chain APS-C. In other words, if they do a pro body, and if they do 'full frame' they would be less likely to do an APS-C size pro body. That means that not all, but some (some of which I own) lenses would be incompatible, or at least less compatible than on the current system. This also means a bigger, heavier camera, and bigger heavier lenses for equivalent focal length and speed. I don't know about anyone else, but I'll take the K20D and the DA* 50-135mm over a D3 with a 70-200mm f2.8 just based on size alone. Besides that, I'm still convinced that sensor technology will continue to allow better and better images at higher ISO's from smaller and smaller sensors.
I've heard it predicted before, and I'll jump on the band wagon to support the idea, that APS-C will take the role of 35mm film in the past, and that 'FF' will take the place of smaller medium formats like 6x4.5.
I'd rather have all the pro features in the smallest possible body, and have my 400mm lens have 14mp or better resolution at an equivalent to 600mm focal length on the 'FF.'
But that's probably just me...