Since this question was originally asked, more options that might give a better solution have become available. Specifically, the Roll20 online virtual tabletop.
With Roll20 you can invite people by email address and they can play as their character while in-game. Some tools make it much easier even for new users, including irc style chat, voice chat and even video. There is also the option of bringing up the interface while attached to Google hangouts which allows even more flexibility with methods of communication, and for players who might need to come and go a lot (hangouts allows you to 'hold' a session and come back to it at will, the session is never actually deleted).
Roll20 is also a GM's best friend once you figure out the idiosyncracies of the interface. Even playing an 'adhoc' game where you don't necessarilly have all your ideas fleshed out can work well because while the players are discussing what their next move might be you can actually be creating a new screen to move them onto in the GM view.
Tokens, maps and various imagery are a cinch to add to the virtual tabletop even if you don't have any assets prebuilt. Just use the built in search tool and drag and drop the resource you want to use to the virtual tabletop.
Want some mood music to enhance your game? There's a menu for that as well. Search for something like 'dungeon' and it goes out to soundcloud and filters the music their using some built-in algorithms that check to make sure the tunes are appropriate as background without lots of crazy explosions but more of a consistent harmonious sound.
There is more to the Roll20 virtual tabletop than I can cover here but I definitely encourage anyone who needs to hold a game online to check them out.