Gizmo actually does voice and *video* between tablets and PCs now. I used it today between an N800/OS2008 to a Vista Ultimate machine. Really pretty decent for their first attempt.

Basically IM = text "chat" in the Nokia world, and there are really 2 ways to get at the popular IM systems out there (like ICQ, MSN, AIM, etc). One way is by installing Pidgin (a popular linux IM client) and its plugins. The other way is using a Jabber server that has various IM gateways installed. I prefer the Jabber route because it's better intergrated into the OS (and I can run my own private Jabber server). I installed Pidgin just in case I ran into trouble Jabber one day, but it never happed

For voice you have options with Gizmo, Skype (again soon), GTalk, Jabber or SIP. Gizmo and Skype are available via their respective installers. GTalk, Jabber and SIP are more integrated into the OS. Any of these can do voice from N8xx to N8xx, or N8xx to PC. Gizmo, Skype and SIP would allow calls to phones as well.

Video is a different story. N8xx to N8xx video is possible via Gizmo, GTalk or Jabber. (GTalk is actually just Google's branded Jabber service)

Video From N8xx to PC can be done via Gizmo currently and supposedly via SIP also (per a Nokia blog comment) although I haven't had a chance to try that out yet.

Technically it could also be done via Jabber if anyone could just find a PC Jabber client that supports "jingle-video". From some reason there don't seem to be any yet.

That should be enough confusion to start.