Oh yeah, I'm running CS3 on an iBook G4.
You might now have two versions of Minion Pro installed and they're conflicting.
You should also try clearing the font caches, and search for and delete all adobefnt*.lst files where the * is a two-digit number.
Hmm, the new font I installed is Copperplate Gothic T., not sure where a second version of Minion Pro would've come from?
How does one clear the font caches?
Just deleted adobefnt11.lst...
You only found one file? I think the ones that ID uses are adobefnt14.lst, and there should be dozens if you run a file search.
You can find directions in http://helpx.adobe.com/x-productkb/global/troubleshoot-fonts-mac-os-x.html item 6 for clearing the font caches, or you can use a utility like Cocktail, or possibly your font manager, if you have one. When all else fails, try google.