One consideration that should really be accounted for is where to have these adventures. It seems to me, as a D&D player myself, that utilizing the forums would drastically slow down the flow of the adventures. D&D uses tons of tiny little scenarios, and without instant communication you lose alot of what makes these kinds of games so much fun and involving.
Using instant messengers easily solves this problem, though the problem there would be setting up chat rooms and making sure the chat doesn't crowd are both very hard to do in IMs.
Just food for thought