There’s a script on Adobe Labs to extract as JPEGs the thumbnails and previews that you see in Lightroom 4:

Sometimes, lost or deleted photos in Adobe Photoshop Lightroom 4 still display a preview. This script allows you to extract and save those previews.

The script extracts previews for all the images that are selected at the time you run the script. It extracts poster frames for any video clips that are selected at the time. You can choose a location for these files. The previews are in JPEG format, and the name contains the size of the image, for example: _MG_0233-360×240.jpg.

Also see Jeffrey’s “Extract Cached Image Previews” Lightroom Plugin.