Apparently org.freedesktop.DBus.GetConnectionUnixProcessID is implemented in the reference C server implementation and wrapped in dbus-sharp and the java bindings. However it is neither documented in the spec nor is it wrapped in the reference C client implementation.
Given that this is used by lots of software these days it would be good if this could be reflected by the spec and in the reference C client implementation.
I documented this in the spec a while back; but it's still not bound in libdbus.
Patches welcome (it shouldn't be too hard to do), but not a priority for me compared with the stack of bugs we have here :-)
Now that the best way to get credentials is GetConnectionCredentials() (which gets uid, pid and LSM context in one round-trip), I don't think we should wrap the less useful function.