We currently have all our machines still on Windows XP and have only just upgraded some with Office 2007.
Budget dollars just have been too cost prohibitive to upgrade all 1,000 of our workstations. At some point due to licensing and availability we will be forced to upgrade.
The one thing I’ve noticed difficulty with right off the bat has been the ability to connect via RDP to a Windows Server 2008 box. Without some registry hacks/mods in XP this can be problematic if the server is set to the newest (and more secure) RDP protocol. Windows 7 doesn’t have this issue.