I used to access Yahoo through Jabber on my old
1200, surprised the hell out of my friends. It
didn't quite work 100% (I didn't show up to my
buddies as "available" IIRC), but definately a
nice and capable program.
Ah, that could be a limiting factor, though Jabber is *supposed* to be able to pass that sort of metadata/state information through if the transport is smart enough... (I notice there might be a new libyahoo or something, which the Jabber transports probably depend on... so maybe it's 'fixed' or improved again. Biggest problem I had - using various UNIX clients, maybe 3 months ago - was false positives from the transport I was using; it was hard to say when I'd properly logged in to the transport, and/or if the transport was actually properly talking to Yahoo.)
The real solution is to convince all your friends to ditch the proprietary crap (and efforts to track it - note that I'm not very hot on Trillian, considering it's a 'band-aid' over the centralized services' attempts at user lock-in) and move to pure/native Jabber.

For those looking for some crossplatform action (AmigaOne or Pegasos *NIX users, anyone with Windows, MacOS or anything else with TCL/Tk), I may as well mention
Tkabber and
The Coccinella, both of which look nice, even though I've never tried them. (Wish I'd found them before I installed Qt just for
Psi, which just doesn't float my boat for some reason. Too much effort put into looking like ICQ.)