Just an update folks. I'm happy to announce, after fighting with PHP for several hours (gave up, got some work done, then went to sleep), and then again for a couple hours this afternoon, the hpHosts forums and fSpamlist blog are back online.
Annoyingly, in the end I had to completely rip PHP out of the server, and install an older version (i.e. the one that was working just fine and dandy until yours truly thought "Ooooh, a new version"). I've still no idea why this particular server hated the new build, given it's working fine on my development machine, and the other two servers it's installed on, but it's working again now, which is the important part.
The server is in dire need of re-build anyway, so I'm going to wait until I get a new machine (buying one off my brother), move them over to that one temporarily, then re-build the current one.