We started our Win7 upgrade (from XP Pro on all stations) last may, when we began budgeting for the fiscal year. We looked at every system in the org (about 175), determined which needed replacing and which just needed memory upgrades, and secured the capital to do headquarters this year and our nine regional offices next year. We plan to do the upgrade in the spring; we'll begin testing in IT now, but we'll wait for a service pack before going live. With Microsoft, we *always* wait for a service pack.
The main justification for going from XP to Windows7 is the improved granularity of the security model. Windows up to and including XP is all or nothing: either our users are local administrators and adware/spyware can freely install itself when encountered; or they're not administrators, and they have to call IT in order to install a Firefox plugin. I call that a no-win situation. Upgrading our systems to the point where we can run >XP took enough years to allow us to bypass Vista; and we'll benefit from the UI improvements in Windows7 (notably, the removal of the excessive, annoying nagging "feature" that Vista called user security).
Luckily, no home Windows licenses here; Symantec Ghost in place to facilitate the upgrade; and a hardware first approach that i advise everyone to take. Windows 7 is Vista with a facelift; you'll cream people who are already slow with XP if you don't give them newer machines (preferable dual core processors) and at least 4GB RAM.
Mind you, I have a spring computer upgrade planned at home, too, and we're going to ditch the two remaining Windows boxes that my wife and kid use and go all Mac. Wish that was an option at the office. :)