If you get the maya 2009 disk (which you should unless its download only) it comes with 32 bit for Windows XP/vista and OSx 10.5.2 and above, and 64 bit for Windows XP 64/vista 64, and linux, currently there's no version of Maya for 64 bit OSX (dont know if OSX is 64 bit as yet) so your licence should cover whatever you install onto.

The general (or what I have to do) is e-mail autodesk with the ethernet address of the PC i'm installing on and they send the licence through to activate it, same to move the licence over to another PC, though there was a guy in the basics forum who said that he had it going on multiple machines.