Back in Lightroom 2 days I had a plugin to extract JPGs from a Lightroom catalog's preview cache, something really useful when trying to recover something if you had somehow lost the master images. It wasn't my most popular plugin, obviously, because folks would use it only when trying to recover from the double disaster of losing their images and having no backups.
It was a lot of work and helped perhaps at best a few dozen people, but well worth the effort to help folks trying to recover years worth of memories. It was my only plugin to have no registration hangups to get in the way of a panicked user.
Unfortunately, changes under the hood in Lightroom 3 caused it to stop working, so the plugin hadn't received a meaningful update since 2009, and faded into uselessness as folks migrated to Lr3 and Lr4.
But with a little under-the-hood help from Adobe, I've been able to get it to work for Lightroom 4. Lightroom 3 is still (and will always be) unsupported, but in theory you should be able to upgrade an Lr3 catalog to Lr4, then use this plugin extract the previews.
I hope you never need it, but if you do:
For Lr4 it now also extracts video thumbnail frames (which are quite tiny so probably of no practical use) if they exist.