We have start trying out Greenhopper's Scrum rapidboard this week and it looks great.
We have 2 different teams which have there own sprint. I have created a different Scrum rapidboard for each team. It is possible to plan the productbacklog for each team, but it is not possible to start a sprint for each team because GreenHopper is limited to 1 active sprint simultaneously.
It would be better to limit the active sprints for each Rapidboard and not over all Scrum rapidboards.