Since Apple writes software that supports very old Windows operating systems from six or more years ago, but does not support its own Mac OS systems from just three or so years ago, the answer is to run Virtual PC on your older Mac, and run Windows XP service pack 2 or higher on it. There, thanks to Windows, you can install iTunes 8 with no problems. You'll have to transfer your library over to the windows side, and you'll have to launch Virtual PC every time you want to update your iPod -- but at least you can use a new generation iPod on an older Mac.
Sad, but it works.

