So, just to verify the obvious: I don't have it installed in a read-only filesystem. This is in the same location as I had RC7, and RC7 worked fine. (And still works fine, too; just double-checked that while I was at it.)
If I run the application by double-clicking on its icon, then attempting to save nets me things like:
Error creating new game:
Error saving game. See starsector.log for details.
Error message:
[../../../saves/save_AresRoy_2302674571779981268/campaign.xml.inprogress (Read-only file system)]
Except, of course, that there is no starsector.log, because it couldn't make that, either.
So I opened up terminal, cded all the way in to the MacOS folder, and ran ./starsector_mac.sh directly, figuring that'd give me a log even if it couldn't write starsector.log. Of course, in the best tradition of a proper heisenbug*, when run this way, the game works fine - starsector.log gets generated, the game gets saved, no problems.
* A heisenbug is the computer-programming equivalent of classic quantum behavior: a bug that, when you attempt to look at it in detail, goes away.