Christoper Null asks "Why Aren't You Upgrading to Vista?", offering some possible reasons.
One reason he doesn't mention - arguably one more compelling than any of those he does - is there's an alternative that's already far better and improving much faster: Mac OS X.
In other Microsoft-related news, the U.S. Dept. of Transportation has declared "an indefinite moratorium" on upgrades to Vista, IE7, and Office 2007. Their reasons for this decision are thoroughly pragmatic, haivng to do with incompatibility with applcations currently in use, cost, and the fact that they are planning to move to a new headquarters.