I wouldn't.
If it's OK for you, no problem here.
I like wide angles, and from what I've seen the Canons wide angles on full frame is not very good. I don't think the full frame Pentax lenses will do much better, so they will have to make new, larger and more expensive wide angles anyway. The vignetting of my 14mm in *istD is better than the vignetting I get with the A20mm on full frame with film (and it will probably be worse with a sensor).
Isn't Pentax 35mm wide angle selection wider than 645?
So the best combination for me would be the small APS dslrs for ordinary use and a large 645D for the special occations, just like I the combination of MF and small format today.
For many photographers, the best combination could be FF for studio and APS for field use. I'd do that, if I could, but I'd hate to have two different mounts. So 350 + 1Ds MkII is fine, while *istDS + nothing doesn't fit well.
IMHO, *istD + 645D is not better than combining *istD + EOS 1Ds MkII.
Another question is why pentax would want to make a camera that competes with its own 645D camera. It is expensive enough to develop one of them.
Perhaps for selling it, instead of letting most people pick a 16MP Canon or a 12MP Nikon.
Dario
