Jeff Atwood give us the skinny on a "official, supported operation directly from Microsoft" to "rearm" your 30 day trial 3 additional times, for a grand total of 120 days.
To do it, open a command prompt as an administrator, and enter the command slmgr -rearm.
So those of you sitting on the fence about your Vista purchase, you have a bit of a reprieve.
[via Buck Hodges]