Those of you contemplating the upgrade; consider the OEM version as well.
There are four major differences between the OEM version and the upgrade version.
OEM can only do a clean install, where the upgrade was designed to only upgrade.
OEM is cheaper than upgrade. www.newegg.com currently sells Vista Ultimate OEM for $199 - About $30 less than the upgrade.
OEM's license states the install is tied to the system board and can never be transferred to another system board. It would appear Microsoft is not enforcing this so far which makes OEM a better deal than upgrade as long as you don't mind a clean install...
The OEM license does not provide support from Microsoft. The system builder or end consumer are responsible for their own support.