George,
I do not administer large numbers of desktops, but I have been installing Ubuntu on a dozen machines for two charities. I also teach business applications at a university north and east of you. Let me add three items to the thread: business applications; ease of software installation, and three issues to anticipate.
APPLICATIONS--Your users are going to be in for a surprise. The open-source business tools are robust, feature rich, and almost identical in look and feel to Win-based tools. For example, Open Office has most of what a power user would need in a business suite. I have not yet evaluated Evolution, an email/calendar/PIM, but there is good support for synching to a Palm. You can get a number of other more specialised business apps from open source download and link sites; e.g. desktop publishing, vector graphics, image processing, CAD, image editing, project management, mind mapping, PDF creation, and so on. In fact, it might be a good time to introduce some of non OS-based wealth available via Web 2.0.
INSTALLATION--For my charities, I have been doing clean installs of WinXP and uBuntu. The differences could not be more dramatic. WinXP takes five to six hours to install, secure with patches and anti-malware, and finally load up with a full suite of applications. The Ubuntu install takes less than 30 minutes, and that time includes creating two extra partitions for a dual-boot OS system as well as patching and toping off with applications. The most jaw-dropping part is the support for old hardware. WinXP seems to have trouble with all kinds of older peripherals: keyboards, printers, MS's own mice, and so on. However, uBuntu recognizes and automatically installs everything. The best part is its ability to run like a demon in much less RAM. Other less-graphical distros do even better.
CAVEAT--I am long in the tooth, so UNIX file systems do not cause terror. However, they could be little rich for the average user. Moreover, my distro does not play well with NTFS, and looking for a Win volume in "Media" is not going to be a natural. Hence you should probably come up with solutions to two file-system problems. Create some version of "my docs" on the desktop, and map any local or remote volumes to it. Second, provide a network-centric solution as well, such as WebDAV or Samba-based system for storage.
Let me close, with one final caveat about change management. Busy people do not like change, even if it saves the firm boat loads of money. Their issues are pragmatic. Do the new tools save me time and effort? Will they make me more effective or efficient? Learning something new does neither immediately. Hence shoot for quick wins or value adds. Teach them how the business applications do what they need. Show them where they can tweak defaults to get what they had or want. Do a little consulting in advance to see if either new tools or a new approach can deliver business-process efficiencies or greater task effectiveness. Maybe teach users how to have fun with some of the new tools. For example, everyone is going to need to tweak X-mas photos. The GIMP could let them do a lot more. Hence sponsor a Flickr contest. Above all, cultivate the support of executive officers and team leaders.
Buena suerte.