GP is spot on. Now I haven't used RPG Tonight's VT so I can't vouch for its capabilities, but I would recommend that you at least check it out since they are sponsoring this.

I can speak for MapTool, Battlegrounds and OpenRPG though. They all have the ability to add grids, text, and objects that PCs are likely to move or destroy so keep that in mind as you create the map.

GP, has posted some of his maps at the MapTools forums under User Creations so if you want to see what you are likely up against you might want to check those out.