Alex,
A quick Google search indicates that it is possible to interface with Project Server over web services, using the Project Xml Data Interchange format. If that is the case, you should be able to simply consume these WS’s directly from BizTalk, either via and orchestration or through a messaging only scenario.
Not the best answer I know, but hopefully something to get you started with! I imagine that other members of the community will have experience integrating with Project Server.
Regards, Nick.