In the January 2008 Edition of Maximum PC, page 45 there is a cool hack: How to delay the Vista Activation for at least a year - This is right out of the magazine:
To reset the timer to 30 day, open a command window in Administrative mode (At the start typ cmd, then press Ctrl-Shift-Enter) - then type slmgr -rearm. This starts the 30-day countdown again. It does this no matter how much time is left on the original countdown. You can do this 3 times for a total of 120 days. Then it will not work anymore.
But, I guess you can give yourself another 240 days by running regedit, go to HKEY_LOCAL_MACHINE\SOFTWARE\MICROSOFT\WindowsNT\SL. Then it the right pane, right click SkipRearm and click MODIFY. Change the 0 to a 1.
Now the magazine says that Microsoft might fix or change this through an update, so know that, and you know the rules about modifying the registry - do it at your own risk ... blah, blah, blah.