For a while I’ve been having a problem with this site where the theme would spontaneously switch to the default theme at random times. Since it started happening around the same time I switched to DreamHost’s one-click install, I attributed it to some monitoring process they were using or the WP-Cache plugin, which was also installed. I worked around it by replacing the theme folder named ‘default’ with a symbolic theme to the actual theme I use. Today Aaron mentioned having the same problem and he doesn’t use DreamHost.
Searching the WordPress support forums turned up this article, attributing it to Alex King’s WP-Mobile plugin being installed incorrectly. However, I don’t use that plugin but I do use David Harper’s Ultimate Gamers’ Pack, which reformats the page for viewing on a Wii, PSP, or DS-Lite.
It’s basically the same problem – I didn’t install it correctly. Each plugin includes a theme folder, which needs to go in the themes folder rather than the plugins folder where I placed them. As soon as someone accesses the site with one of those devices, it will fail to access the theme, which will cause WordPress to revert to the default theme.