> I just wondered if I could access the same 8.4.2 server from the Windows > partition (XP via "Bootcamp") as I do from the Mac partition on my Mac?
Assuming both virtual machines (or the virtual machine and the host) are up at the same time, it's more of a networking issue that anything else. As long as the pg_hba.conf and postgresql.conf files have entries allowing outside machines to connect via TCP/IP you should be able to just point your windows partition over to the IP of the Mac partition and be set.
I haven't used Bootcamp for a few years, but I believe that it is not platform for running concurrent virtual machines, but rather a method of "dual booting" Windows and Mac OS. Meaning, that either one or the other would be running at any given point of time.