I just did a cross-platform upgrade from Photoshop CS2 for Windows to Photoshop CS3 for Mac. Just call Adobe, give them your serial number and payment method, fill-out a certification that you are agreeing to give-up the Windows software for the Mac (which you then fax to them), and they'll send to you the full-install disks for the Mac.
I believe they allow you three cross platform swaps (not counting up grades) for free.