Thank you both.
Yes, with the 1ds3, it seems to be a Tag, and you can back out afterwards, if you need bleed. But I still think I'd prefer the Nikon version.
Do you see 4x5 crop thru the viewfinder, or do you have to install a special focusing screen? The upside of the Canon approach, to me, is that with the blackout screen, you ONLY see 4x5 in the viewfinder. You're composing 4x5...