Before this sounds like a rant, I'd like to first say that I think PulseAudio is a very nice concept, from a technical perspective. It's about time Linux got a manageable sound subsystem, and some of the features---such as network streaming and synchronised playback---are quite impressive.
However, one thing that really irks me is that, as deployed by major distributions, it appears to enforce the narrow-minded view that a desktop is used by only one person, and that sound is only ever going to play when somebody is logged in to a graphical session. Anybody who uses MPD has probably experienced the pain caused by this already. Unfortunately my experience so far with Ubuntu Intrepid is that trying to change this results in even more pain. Having set up a system-wide instance I'm now in the position where the first time I attempt to play sound works correctly, but any subsequent attempts fail until I kill a per-session PulseAudio instance that has magically spawned (even though I've set it not to). Once the per-session instance is dead, I have no more problems and everything works perfectly until the next time I log in.
I'm currently trialling Fedora 10 on my desktop, so I'll update this post if I have any more luck there. Feel free to correct me in the comments if I'm just being stupid or missing something!
Update: I've since fixed this problem.