That's why I was considering something along the same lines as BM—or, perhaps more accurately, like WI. Something where each player can log on, do what they have to do, and when everyone's gotten their stuff in, things can move along.
The basic idea would be that the DM has control over everything but the characters' own persons—and that includes having control over the characters' positions, so they can't go gadding about without him being able to react—and players essentially "propose" actions, which the DM can then confirm, modify, or deny. Everyone, of course, can talk all the time. Talking is a free action 
So if all the people are online at once, things can move pretty quickly, with everyone moving, the DM confirming actions, and the whole play flowing fast. If people can't get online together for a while, they can still play at their own pace, and the DM can make an executive decision to just let people who don't show up for a week follow the party mutely.
By the way, this is exactly how Fantasy Grounds 2 and MapTool work (two popular tools for dnd-over-the-net).
