I have been having this problem ever since i am running WP as a blog host. My theme changes automatically to the default WP theme after a couple of days of me changing it to something else. This has been happening with a couple of difference themes.
I have looked for support around here but no one seems to be having a definite answer. One suggestion was to change the name of theme that i want, to 'default', and delete the default WP theme.
I did that, but when time came for the theme to change to the default WP, it could not find it since i had deleted it) and my site just displayed a blank page instead. So essentially it was not just looking for the name 'default' but the attributes as well.
I think this is such a big issue that there has to be a fix for it. At the same time, I think that not everyone is having this issue or else there would be resolution for this.
Btw, does anyone know if this issue is fixed in 2.2?