Anne Fisher of Fortune has a very good article on making virtual teams more effective at IBM. In addition to the points that Fisher makes, managers should consider what I call the "three Rs" of successful virtual teams.
First, what are the ground RULES by which the teams will operate? How will they communicate with one another? What are the shared norms of behavior? What will be expected of folks in terms of availability, response times, etc.?
Second, what are the ROLES of each team member? Making sure each person has a clearly defined role can be especially critical for a virtual team.
Finally, what are the RESPONSIBILITIES of each member? How is the task divided among the members? What are the interim deliverables? One challenge with any team is the diffusion of responsibility: when everyone is responsible, no one is responsible. Making sure accountability is clear proves especially critical with virtual teams.