Lightroom previews should be stored as individual files in the "Lightroom 3 Catalog Previews.lrdata" folder. This folder is kind of unruly, with two levels of folders...single hexadecimal numbered folders from 0-F, subleveled by quad hexadecimal numbered folders filled with folders that cover the parent folders hex value (i.e. folder "0" contains folders like "00BC", while folder "F" contains folders like "F0A1".) Within each subfolder is an .lrprev file. These files contain previews of various sizes, in JPEG format (at least, I've only ever seen JPEG).

You can extract these images with the tool BitmapRip. Simply copy all the .lrprev files into a temporary folder, say C:\LRPreviews. Copy the BitmapRip executable to this folder, and at a command line run the following:

for %1 in (*.lrprev) do BitmapRip %1

All previews should be extracted into the same folder. If you had full-size previews, you should have full-sized JPEG copies. Sadly, I don't think it is possible to retrieve RAW versions, and I have never seen any TIFF versions. But, at least you'll have some JPEGS.

(Original source for this technique: http://www.flickr.com/groups/adobe_lightroom/discuss/72157603510380245/)