Don't have one of the devices myself but in following threads and things last I heard the video chatting capabilities were disabled because they did not work properly in current firmware/software; something to do with codec negotiation issues or somethin. Once those are resolved I'm sure there will be at least the same level of support as for N8x0 series which was google talk and SIP. Skype is up to skype, and since the first days of the N800 they never bothered to build video support in for whatever proprietary reason (though I heard they're moving their linux client to open source soon, so maybe that can change); other protocols are just a question of whether they support video in open source implementations I believe.

For the record I've been using video calls to PCs AND other devices on my N800 using SIP for 2 years now, so don't see a practical reason that won't be the case on N900 once codec bug gets fixed.