Just to add some perspective (old) , the older tablets N810 could do video call (using Gizmo or using the built in Gtalk to another tablet with Gtalk) over 3G.

Of course you did need a very good and fast connection (I had 3.5G speeds) and sometimes it was a bit spotty - but you could do it.

Dont know the specifics for the N900 though as yet.