After having a closer look at the Message Exchange Patterns in the WSDL specification, however, I found that they weren't described in a very intelligible way (despite the fact that they are actually quite simple).
To make sure I understood the patterns and in order to document how I implemented them in Soya, I drew a state diagram for each pattern. Each state diagram represents one MEP from the perspective of one service.
Clicking on the image below will open a high-res image of the state machines for each MEP. Alternatively, you can also download the image as a PDF document for easier printing.

Please let me know if you find any errors in the diagrams, so I can fix them in both Soya and the image.