It doesn't add to Steam but then again, if ActiBlizzard can do it, then more power to them.
In any case, be it Steam, Blizzard's online download service or Direct2Drive on the PC side, or PSN, Xbox Live and Nintendo's VC on the console side, it's very telling: digital distribution is here to stay.
This should be great for gamers because digital distribution doesn't carry the overhead of retail, which in the long run should bring prices down (I hope).