Jeff - do you prefer straight lines or the "bent" versions in Rob's diagram? I prefer the latter, but it's probably good to check in on how people feel about it.
My current models use straight lines, but I can see the use of bent lines as well. I would not expect too complex of a diagram however. If I saw a diagram that looked like a BPMN2 process definition with lots of nodes and interconnections, I would worry about this service composition. It seems like service compositions should be fairly straightforward and easy to read. If no, maybe the service is trying to do too much.
I definitely agree with the comment on complexity and how a really busy diagram can be a warning sign. The main objective here is for connections and component arrangement to not overwhelm the information being presented. Those straight lines annoy me big time, so bent lines allow me to focus on the important details. :-)