Usage and download: http://userscripts.org/scripts/show/5565
Blog post with some details on adjusting it for unsupported layouts: http://henrik.nyh.se/2006/09/10/preview-images-from-within-livejournal-cuts/
Example lj-cuts follow.
Since there is an image (the screenshot above) preceding this cut, you won't get a preview at the default settings.
This cut is not preceded by an image, so you will get a preview.