Hello again, all. I'm pleased to report that after some investigation, I've managed to fix this problem. I'll detail the problem here so hopefully anyone else who runs across this can fix it.

The problem was, as I thought, with maemo-pc-connectivity. Through what is I assume a bug, it seems to have managed to manipulate my network configuration files into a state which meant that the local loopback interface had disappeared entirely.

Somehow the following files were broken and needed fixing:

/etc/resolv.conf
Contained nothing but a comment line. I restored this to its original state from another N900.

/etc/network/interfaces
Contained just USB networking config from pc-connectivity-manager. I re-added the lines for lo and eth0 from a working N900 at the top.

Rebooting after making these changes caused my networking (and sound) to reappear. I hope this can be of some use to anyone who runs into similar problems.

If anyone would like more information to track down the cause of the problem, just shout.