I used to work for a company who had 100k plus NT desktops. Upgrading to another OS from that takes ALOT of planning. You can't just "roll-out"
What kinda puts people off are the number of companies who blew alot of money rolling out Win2k, only then to have XP Pro walked into the door with it's matching server family.
Corporations don't just "upgrade" to the latest OS like core enthuiasts do. There is far too much money at stake if it goes south. Only upgrade if you need to. Infact, most OS installs NEVER get changed unless there is a fundamental hardware fault on the rig.
Every new machine built for the small company I work for, is still having 2K as standard. No need for XP and it's more costly structure (plus, we are too small for volume license agreement and activating is a hassle, something ALOT of small business IT companies have complained about).