Is there any overlap between MS WF and NServiceBus, in terms of workflow specifically? Is there a need to use one if the other is already in use? How well do they play together?
NServiceBus supports workflows through "sagas", http://nservicebus.com/Sagas.aspx
The key difference is that Sagas is easier to unit test and that you don't have to worry about consistency since NSB's underlying use of queues and DTC makes sure that failed steps are retried automatically.
Udi has a podcast discussing all this:
http://www.udidahan.com/2007/10/23/podcast-durable-services-with-wcf-wf-and-nservicebus/
In short: If you use NSB there would be no need IMO for MS WF.