If software project managers are diplomats -- and I think most project management roles see them this way -- then there are at least two distinct flavors: heralds (low level diplomats) and negotiators (high level).
The difference is sometimes imposed by the project/work situation, but more frequently appears to be self-imposed by the project manager.
The herald project manager sees his or her role as a courier bringing messages back and forth between various parties. The parties may be friendly or hostile, and the communications may be straightforward, complex, or threatening. But if the herald gets the info to the recipient in a timely way, his job is done and he expects to pass unmolested.
The negotiator project manager, on the other hand, sees his role as keeping everyone on the same page about decisions and outcomes. If (when) parties are not in agreement, the negotiator tries to bring about sufficient discussion and confrontation that something can be agreed upon.
The project manager rarely has direct authority over multiple parties in the project (e.g., engineering, product management, and marketing). He can, perhaps, control one closely aligned party through resource allocation. In general, though, I've seen more of a carrot-and-stick approach:
The PM has opportunities (e.g., a recurring meeting) where he holds the floor and escalates all of the known issues, so that everyone is painfully aware of the potential problems. He then reminds everyone of all the negative consequences coming down the pike for the project, if key decisions and compromises are not made. He also points out how well things will turn out for everyone if the project can reach completion on time, on budget, and with everyone more or less happy with what was done.
With enough persistence, it is usually possible to keep things on track. How? The negotiator's secret weapon in the business arena is his willingness to make people consider negative possibilities that they are not likely to raise on their own, and to make it seem matter-of-fact. Business groups (at least in the U.S.) are extremely uncomfortable thinking about and planning for negative outcomes. So the project manager gets to make a whole room full of normally confident people very uncomfortable. In order -- literally -- to alleviate this discomfort, they start to talk and become a bit more flexible.
It is clear that the negotiator PM is doing the heavy lifting, while the herald PM is a glorified secretary. The herald may be useful in a large project where so much information is flying around that it's worth having full time staff to keep it straight. He's holding off entropy but not solving big problems. The negotiator feels like his success depends on getting people to work together for success, which is a bigger challenge and produces bigger rewards.
When you are choosing, hiring, or staffing project managers for software projects, keep this dichotomy in mind. Unless your world is all rainbows and unicorns, you probably need a negotiator, and getting this sort of PM will pay off handsomely. You can supplement him, if the project is really big, with a herald PM to "do the paperwork."
But if you enter a challenging project with nothing but a herald PM, you are increasing your project risk considerably.