I had UMA on a blackberry 8220 flip -- the only thing I miss from it.... I would love to see this on the n900.

I found this quite helpful and informative regarding UMA.
UMA Demystified: Inside UMA-enabled dual-mode handsets from Mobile Handset DesignLine

http://www.mobilehandsetdesignline.com/howto/205906491

It was written by Steve Shaw, Kineto Wireless. They seem to provide the software client software for some handsets.

From my read it is basically voip over vpn with some handover stuff... pretty neat.