I’m a Unix sysadmin. I got a new work laptop today, still on XP. I asked the IT guys if we were in any danger of Vista. They said “Nope, XP is supported for years yet!” And we all exhaled.
We have worked out that if we are ever threatened with Vista, we promptly (a) pump up the Gutmann (b) write a whole pile of in-house apps for ourselves that only work on XP. The latter already worked wonderfully for us in making an instant business case for staying on Firefox — make sure your in-house web apps are written for Firefox and SeaMonkey, and specifically break in IE. (This is easy: just write to standards. We have one vital app for twenty people that was broken for six months in IE with no-one realising …)
So: to stay off Vista, stock up on in-house apps that don’t work on it. Then you have the business case you need.