hey guys. I posted earlier with the cs5 photoshop upgrade not recognizing my cs 4 serial #. wanted to give an update. Turns out the cs5 installer wasn't recognizing my installed copy of cs4 extended as a qualifying product. so after a nice chat with tech support, uninstalling cs4, uninstalling cs5, creating a clean login account, and zapping the PRAM, the same problem persisted. I contacted tech support once again (those chats connect fast!) and they told me the secret command+apple double click on the install screen method. that produced a challenge code, then they gave me the answer, and that circumvented the serial number problem. now cs 5 extended is happily installed. and it only took 2 hours! jeez, this feels like a windows install, it's not supposed to feel like this on a Mac!
anyway, happy endings are good. CS 5 PS is amazing.
Good to know the workaround, Enjoy your product