@jax - nope, don't think they plan on it being used with other systems. If they were to expand later on it would probably only be for their own games as well.
@oldguy - haven't played with other systems than 4e yet - haven't even had time to return to try some more. For now the program will be free with a ddi subscription but that does leave one with 2 big questions:
1. Will they charge later on? - the wording on the website does apply that there will be additional cost later on.
2. Will it be required for ALL players to be a ddi subscriber to play? They don't say anything about that yet - however my gut feeling says yes. There has long been an "understanding" that a ddi subscription to ddi was for the GM and up to 4 players (since you were able to update "character builder" 5 times a month but Wizard always only came with one monthly update. I believe that they want to change that now and have all players subscribe too. On their webpage they tell people to subscribe to gain access to the beta test (clues of what to come? YES)

What will that mean - imho it spells the death of the program after the initial beta opening is over, of course there will always be some people who are willing to pay to play - but consider this, if Wizards will ask for more money to gain access to the gaming table (or a separate subscription) then we're suddenly talking people having to cough up x-dollar (my guess would be 5) every month. The closest competitors would be MapTools which is free and Fantasy Grounds (nicer interface) which costs $25 for a player license (one time fee). I think with fees in addition to the ddi fee people will drop out of using the program and Wizards will be left with a small group of players, which will not give them the funds to keep developing the program, so the system will stagnate and finally end.