Can you find the thread?
The fact is, though, that it does not inherit from the main site. It inherits from the subsite below Main Site, but above the subsite that I'm mounting the application to.
Main site - subsite 1 - subsite 2 - forums
Forums inherits from subsite 1, not Main site, not subsite 2!
Hm. Trying it out now on a fresh OpenACS install, everything inherits from Main Site, which is at least consistent. Something's probably been fixed in the last week.