Hey all, I'm KaKaRoTo, aMSN developer.
Just want to clarify what I said to gadgety since it seems people are confused.
aMSN has supported the 'webcam feature' for years now, it's an MSN proprietary protocol that allows the webcam to be sent or received, which means a unidirectional video only call. You can of course send and receive the webcam, it just involves two 'send webcam' invites/accepts.
Since a year ago, I got a newer type of video calling feature implemented in aMSN (which was first released as the 0.98.1 version of aMSN a week ago).
The video calling feature is very different, it's not proprietary, it's all standard protocols (SIP, RTP, ICE, TURN...) and it's bidirectional real time audio and video calling. There's also a SIP/RTP bidirectional audio only call feature.
With aMSN, you can now do an audio and video call on your computers, and it works fine (usually, not always), we're using farsight and gstreamer and libnice, which are all bundled by default on the N900. The problem with aMSN and this audio/video SIP call is that the code in aMSN is not able to correctly create/start the pipeline, so it doesn't work. But I only tested this once, so maybe I was just unlucky that day.. maybe it all works fine, I have yet to try it again, but in theory it should have worked from the first try because the N900 has all the dependencies we need...

So that's what I said doesn't work and which I'd work on someday...
What *does* work already is the old proprietary 'webcam' feature. So you could always do a send/receive webcam invitation in amsn, get your webcam call in aMSN, and use your SIP/gtalk/skype/whatever account for an audio call at the same time.

I hope that clears out the confusion.

KaKaRoTo