Forum member shinichi_wtn has developed a homebrew utility that backs up SMS messages on interop-unlocked Windows Phone 7 devices. It cannot restore them to the Messaging Hub yet, but you can import previously exported messages into this SMSBackup app, which then shows them in a human-readable format, sorted by conversation or time stamp. Not perfect, but better than completely losing your texts or viewing them in a crude XML format.
Basically, it regularly saves the “CommsBackup.xml” file, which contains all SMS messages older than a day, and then copies the file either to the app’s Isolated Storage folder or uploads them to SkyDrive, so you can access it easily on your PC. Credit for finding this file goes to forum member Thunder_, whom you may remember as the developer of Office File Uploader.
If your Windows Phone is interop-unlocked, head over to the forum thread to download this app, and be sure to leave some feedback!_________