I did some looking around, and settled on Ignite Realtime's Jabber-compliant server OpenFire. It has all of the features I think they want, plus it's open source (GPL) and multi-platform (Linux / Windows / Mac). I chose to install it on one of our Windows servers for a couple of reasons. First, I had planned on using Active Directory as the user repository (it didn't pan out due to our multi-domain setup), and second because I wanted to attenuate some of the culture shock that might come from my rolling out an open source product in this very Windows-centric shop.
Installation was quick and painless. The only hitch was user-induced: don't reference the server via a DNS alias when you've told the server setup to use the box's canonical name. Ungood.
Client-wise I've been working with Jabber's own Exodus client. It's pretty good, though I don't like the lack of sound-based notifications. I pulled down Pandion, Psi, and Spark (in Windows, Max, and Linux versions) to give the helpdesk folks some options. Any Jabber client should work -- there's a whole roster of them.
After some testing with help from ashacat this looks ready to go. I'll let the helpdesk team kick the tires on Tuesday. Hopefully they'll like what they see.
...And the first petal of my Open Source chrysanthemum has unfurled. ;-)
Edited 'cause I for some reason I just have to have a flowery metaphor.