Ars points out that if you’ve been rocking the CS3 beta of Photoshop, a simple Drag/Drop uninstall or copy of the Full version over top may not do the trick. Adobe offers a script that you should run before installing the official (not-free) version on your system.
It’s unclear whether using something more sophisticated such as AppZapper may also do the trick. My thinking is if Adobe’s made it readily available, might as well take it for a ride and be on the safe side. If you’ve got experience with either method, let us know your findings.