So my question is this: is this a standard thing in a large operation or is this as unreasonable as it feels?

It's a standard thing. Our company is trying to do it, too.

Although they're being a bit more realistic about timing, and letting us software development folks keep certain machines around on the older OS's if we say we need it for whatever software-development reasons we can come up with.

I'm not sure the reason they're doing it. I think it's licensing... i.e., bulk licenses for XP are being purchased from MS. But that would imply that their prior licenses for the older OS's were somehow time-limited or made invalid by the XP purchase. I didn't know they could do that?