Welcome to the club!
I have a thread going to collate all relevant information regarding the terrain locking problem.
To answer your question: No, nobody has a real solution yet.
Here's what we know so far:
At an unspecified point, the editor decides that your terrain needs optimizing. This seems to be the same event that locks it.
Keeping backups means minimal lost work, and easier narrowing down of the exact point where the editor chooses to "optimize". Do a quick archive-backup (using something like winrar) on your map before editing, so you won't lose more than the most recent work.
Norbyte (the ever-present entity on the IRC) has been working on a terrain de-optimizer. Don't hold your breath though, he/she is busy, and making shiny toys for us isn't his/her responsibility.
If you have any other information you can add to the thread, post it here:
http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=551365#Post551365