I didn't get much in the way of advice, so I decided I'd dive in, now that I have my iMac. I have good news, bad news, and good news, in that order:
(1) Cakewalk SONAR 5 Producer and Project 5 version 1 will indeed install correctly in Windows XP under Bootcamp. The software itself performs as one might expect.
(2) The iMac's Realtek High Definition Audio hardware, however, doesn't. The latency is horrendous. I cannot emphasize this enough.
HORRENDOUS.
There, that's more apropos. Even updating the Realtek drivers straight from Realtek's site didn't help.
(3) Using external USB or Firewire audio hardware such as the Edirol UA-1A, which I happen to have lying around unused, removes the latency issue and makes Cakewalk SONAR perform acceptably.
Long story short: Want to use Cakewalk SONAR in Bootcamp on an iMac? Avoid using the iMac's internal hardware and go the external audio route, and you should be fine.