Looking for something else when I saw this thread so haven't read the entire thing so thought I would add a few things (that may or may not be covered already) since I use it a lot:

- 3G or wifi doesn't matter even in the US (um, yeah, that's where I usually be, I think) - gtalk is data, not the same as what someone was referring to.

- If you use gtalk and are like me, you are probably signed into your account from multiple devices. Text routes nicely as you know - first client to respond to a new convo is now the primary client until you move to another device (or browser on a device).

- However, voice and voice+video don't 'route' so nicely. I noticed this a long time ago, but haven't spent any time looking into it, but suspect it has to do w/what they are doing at L4. For it to work (at least for me), the TARGET account you are calling can only be logged into one 'place'. So if you have gmail web interface up in firefox and logged into chat w/browser and running whatever client you want AND logged in with your N900 24/7, it won't work. Pre-N900 I started keeping my browsers logged out because of this. I usually have at least 2 running on my main laptop and forget to close on other laptops/desktops so it can be a royal pain to figure out which puter I left a browser running so just easier to not login except with a client.

- Someone mentioned NAT/FW issues - shouldn't be any problems with NAT or PAT (which I assume is really what was being referred to) unless there is a problem with the state table, or playing with stateless stuffs or doing something funky. There are a number of ports it needs. IIRC, they are all UDP (of course).

I've actually used gtalk voice a lot with the N900 (and yes, over 3G too), but haven't used video much. Video is not great w/a very slow refresh rate even if you have a >2Mbps 3[.5]G connection or wifi. I think someone said how great the video is - no idea what he is using, even a nice MBP doing a gtalk video call over a 10Mbps home connection has marginal video with gtalk. I guess I just wouldn't call it great or be that excited about it. BUT, it isn't bad if you are traveling and want to make face contact with someone or you get your mistress and girlfriend their own N900s and you want to sneak a quick peak whilst your wife is on the phone with her boyfriend.

Anyway, that's the only way I've personally gotten it to work (on anything) and it works fairly well. I use it a few times a week (voice, no video) and 99.9% of the time, I am out and about on cell data when I do use it. Oh, and I don't have 2 N900s to test with, but I can't see why it wouldn't work (unless that's been determined already in this thread somewhere). If N900 to N900 wouldn't work, then N900 to something else wouldn't either.