Hey Chris,I signed-up for their Q4 2005 beta release of a Mac OS X version of Meetro.
I don't know if you are extremely open to shameless self-promotion on your blog or not, but I've developed a new type of IM that might strike your readers' collective fancy. It's pretty much exclusively for wifi networks (for now), and is completely proximity/radius-based. Check it out: www.meetro.com -- I'd deeply appreciate any feedback you might have.
Shameless self-promotion is always a good thing, i'm sure the two persons who read my blog won't mind. And it does look interesting.
Meanwhile, if you've got Windows XP, I guess you're good to try out Meetro. Feel free leave your thoughts in comments.