Think of it this way-I'm in two games right now in IRC. IRC was the medium for me to play the game, but not to find it.
Wrong way to think about it. You may have had IRC before, or may have gotten it to play with.
D&D VTT wil be there on DDi. Like an MMO, you install the software and use it to connect to other people. DDi may have a LOT less to install, but same thing. Now for ease of use, most do away with making you learn your IP and trying to connect via direct IP connection, but WANT you to use their servers to connect. This will allow VTT behind a paywall, as well nothing to download but running from the java applet, to be controlled by the company, as well a give people a bigger chance to connect with people without having to know their IP, someone else's IP, where to use that, etc. Like IRC you need to know which server to go to, MMOs do that too, but unlike IRC, there isn't a need for different servers to ahve a lobby.
I am really surprised some people don't get this. Maybe you aren't from the video game area and never really delved into it. Playstation, Microsoft, Nintendo (i think haven't used a Wii) all have some sort of player lobby for their games, whether it is subscription based like XBOX Live, or something free from the company that makes the game.
In order to capture the biggest portion of video gamers that might come over to D&D with the aid of this VTT, there needs to be something that resembles what they know to allow them to get into a game.
WotC forums are pretty much crap, so that is out.
Aside from persistent worlds, one of the big things when NWN first came out was that you could use it as a somewhat automated virtual table top, and it even did the dice rolling via the 3d fighting. With the exception of not having the feel of direct TTRPG, and of course not using 4th edition rules, but 3rd (I think), it is still the best VTT for D&D that would draw in video game players to compete with WoW, but actually play a more traditional PnP RPG. You can be there as a DM with a small party and run closed games, or have open ones.
The key thing is the lobby that lets you connect to people and have the ability to join games with people you don't know.
Anyone can use forums, or telephone or whatever to start, but MANY will jump right in to find people, and that lobby is a key component that will be needed. Even OpenRPG has/had a lobby where you could see servers running. You didn't ned to go to a website, when the software wa running it let you find games to join.
If D&D VTT can't offer that, then people might as well use OpenRPG with Skype for VOIP.