Usage and download: http://userscripts.org/scripts/show/556
Blog post with some details on adjusting it for unsupported layouts: http://henrik.nyh.se/2006/09/10/pre
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.