I need to write requirements/specs for a WCF service interface between our system and a third-party BizTalk server. I'm not quite sure how to do this, so I'm looking for a template. I guess I should at the very least define the interface itself, authentication policy and error handling. Any resources regarding documenting WCF/SOA requirements would be appreciated.
I couldn't find any readily available templates, but I would suggest looking at Interface Control Documents to get some ideas. This type of document is designed to describe the boundary between two or more systems, and seems to be exactly what you are looking for. There are many variations, ranging from a formal specification in textual form to combinations of sequence, deployment, and data flow diagrams.
You can also take a look at a more generic software requirements specification template. The format of the document isn't the important part, but what information is captured is. This includes use cases and the functional and non-functional requirements.
Regardless of the method(s) you choose to capture the requirements, your process methodology will probably dictate what output would be most useful to the development team. In a plan-driven methodology, the expected output would probably be textual documents with appropriate diagrams inserted. In more agile methodologies, the functional requirements of the system would probably be captured as use cases and user stories and any associated diagrams would contain just enough information to increase the understanding of the system.
To the best of my knowledge, there isn't anything unique to the particular technologies you mentioned - requirements and specifications exist at a level above the technologies chosen to meet those requirements.