A better way to conceptualize the Virtual Tabletop is like playing online chess. You use a program, it has a map with pieces you can move stuff around on it, and it also tracks other things (how many moves you've made, what pieces have been taken, et al).
So the Virtual Tabletop will probably be like OpenRPG, Maptools, and other existing software that allows you to play RPGs online.
If this is the case, it'll just be some software that does this:
Has a map program that allows you to move minis around on a board.
Roll dice (via a dice program).
Possibly have something that lets you use character sheets.
Chat interface (basically a chat window where you type stuff in).
Though, since all these features all ready exist in programs I just listed, I'm curious what will set the WotC Virtual Tabletop apart from them.