While cleaning up today I found an interesting piece of information in the Microsoft Action Pack material. It appears that the 30 day grace period of Windows Vista can be extended to 90 days by rearming the installation (you should also disable auto-activation during the installation). This is great for testing purposes.
To rearm Windows start a command-prompt with admin. privileges and run
cscript %windir%system32slmgr.vbs -rearm
The script can also be used to active Windows using the -ipk switch followed by the 25 digit activation code.