Jeff Atwood has written a post on how to extend the Vista grace period to 120 days. It turns out that you can extend it three times for 30 days each, giving you up to 120 days when you count the original 30-day grace period. It seems like a great way to try out Vista Ultimate to see if it's worth the extra money.
If you're on the fence about the impending release of Windows Vista, I recommend trying before you buy. Every Vista DVD includes the ability to install any edition of Vista without a product key. When you install without a product key, you get an automatic 30 day evaluation period.* This probably isn't news to anyone.
What may be news to you, however, is that you can easily extend the 30-day Windows Vista grace period to 120 days. No hacks required. This is an official, supported operation directly from Microsoft.
To extend the grace period another 30 days, simply start a command prompt as Administrator, and issue this command:slmgr -rearm
Reboot for the change to take effect, and voila, you have 30 more days. You can only extend three times, so the total grace period for a Vista evaluation is 120 days. You do, however, need to be careful that you've installed the correct edition of Vista. At the end of that 120 day grace period, you'll have to pony up a license fee for the edition of Vista you've installed.