I recently had to put my primary computer in the shop, and I was temporarily booting my laptop with my external system. I wanted to keep making backups, so I followed the procedure above. It worked, and I was able to keep making backups, but when I tried to switch back to the original iMac, I followed the procedures to return to the old settings but now the only way to bring up the old backups is to browse the drive. The trouble is I don't know if it's because the procedure doesn't work in reverse, or the fact that I didn't try until after I updated to 10.5.2. I suspect something changed in the new version of Time Machine, and some metadata isn't updated so that it automatically recognizes old versions. Fortunately, it claims to still be doing backups, and when I check, the versions are there. Unfortunately, it takes an extra step to get to them.

Just a heads-up, in case someone has done this and is now seeing problems.