Works! Works! Works!

The error wasn't on the N800-side. It was my wireless router. I had multicast disabled on the wireless interface. Enabling it made Avahi work for the "Pidgin to Pidgin" setup between my N800 and the desktop. No downgrading, no hackery. Great! (The only thing is I need to start the Avahi daemon manually, but I'm not gonna change this as long as I'm experimenting... Each daemon not running is a good daemon )

Mhm... Now it's getting hairy. If I understand you correctly, there's no Avahi-support in Telepathy out of the box in OS2008. The part that would add this to Telepathy is Telepathy-Salut. Telepathy-Haze might work, too (as it simply adds whatever Pidgin offers, and Pidgin can connect via Avahi). In order to get it to work, I first need to get through all the hassle described at the rtcomm-homepage (red pill, download installer,...) and then downgrade the now working Avahi-installation to a version that will be compatible with the current rtcomm-stuff.

I think I'll just leave it the way it is... Doesn't seem to be worth it. I can if I want (via Pidgin) and I'd rather not screw up my system to I point where the next SSU might fail.
Still I hope that there'll be better support out of the box in future releases of OS2008. It's just sooo cool!