As far as I know, you are allowed to use it on two machines, as per Adobe's EULA. Me and my brother used Photoshop on one computer. I got a mac, I want to partition my mac with XP and use the second license on the mac.
2.4 Portable or Home Computer Use. Subject to the important restrictions set forth in Section 2.5 below, the primary user of the Computer on which the Software is installed ("Primary User") may install a second copy of the Software for his or her exclusive use on either a portable Computer or a Computer located at his or her home, provided that the Software on the portable or home Computer is not used at the same time as the Software on the primary Computer.
So I can use it one the Mac, as long as it isnt being used on the PC, which is what happens anyway.