I had to register just to reply that we saw the same thing last night. We're on Exch 2003 and moved some users over to a newer existing Exch 2003 hardware platform. (Its been in production for some time with most of our existing BB users.)
This AM, the user next door to me tells me he was looking at his BB as it reset itself to default themes etc. At least we know we're not crazy now.
I've several other users to migrate and now can warn them ahead of time. Unless there's a known way to prevent this?