Did you enter the serial number for Photoshop Elements 11 as well as your qualifying previous version of Photoshop Elements?
Thanks Jeff, but it only asks for the current serial number - Which I know is not right.
I did get it to install, however, by manually going in and deleteing all references to Adobe from my system. The Installer is not handling an exception properly and is failing. Sorry, that's a bug. The offending file apparently was in /library/application support/Adobe. For my purposes, I only have to re-install one additional product to insure that everything still works. The fact that it installed an update without asking for a previous serial number is also a bug - perhaps the same bug not completely cleaned up. My opinion is that they either have to fix the installer or offer a cleaner - maybe there is a cleaner but I did not know about it. I am marking this as complete but the developers clearly have some work here. Perhaps it is not possible to handle every exception - I can accept that also.
Thanks for the update Jeff. There can occassionally be file permission issues which can cause serial numbers to be seen as invalid. From your description it sounds like this may have been what you ran into.