Do the fx1/z1 use the same threading as the other sony camcorders? I already have a few different lens's from my VX2000 that I am thinking about just keeping to use again for when I pick one of these up.

It's doubtful it would work. The VX2000 had 58mm lens threads, the FX1 and Z1 have 72mm threads, so you'll likely see very strong vignetting if you put a 58mm lens on it.

Holy cow! I just realized, aren't the Canon XL1/S/2 lenses 72mm? Wouldn't that be something? ;)

Some of the Canon lenses are 72mm, yes.

Then it becomes a question of, is the glass good enough to adequately resolve a high-definition image? It may very well be, but XL1/XL2 lenses were designed to perform for a standard-def camera, so they may not be "up to snuff" for high-def duty. Some testing is in order...