Just to provide an update, I submitted a ticket earlier today and recently received a response. Some of it seems a little cut-and-paste-y (specifically the bit about this being in the knowledge base, which as far as I can tell it isn't), but it does say they're aware of the issue and working on it. Relevant part of the response to follow.
I am so sorry to hear that you and others are having to experience something such as random rollbacks. We are still working on this. We know it's a problem, and that it's highly inconvenient for our players - we're still working on preventing this from happening in the future!
For now, we do have a suggestion to help minimize your frustration while we get this resolved.
Whenever you accomplish something significant, or simply make a good bit of progress that you'd hate to lose, please log out to the character selection screen and log back in.
We realize this is not a long-term solution. But it should help prevent you from losing hours of gameplay at a time. This works as a sort of 'save game' by preserving your character at the point when you last logged out.
Like I said, we are still working on this, it is one of our top priorities; no one likes losing progress, experience, and loot to something as arbitrary as a server roll-back.