I've been running WordPress for over a year, and have installed various themes and everything has worked fine.
I installed the latest version of black-letterhead (1.2) and a new problem appeared: in the Admin Presentation panel, I select black-letterhead, and it changes to it just fine.
Sometime later, or the next day, I find the site using the default theme. I change it, rinse, lather, repeat. :-)
I've checked file permissions and so forth. Nothing seems to be out of order. I've cleared caches after changes, etc.
Ideas? Where is the information stored about which theme the site should use? I looked at the mySQL tables; doesn't seem to be there, and grepping through all the source code doesn't show any changes either.