The plugin's not working for me. Scrolling down the page does nothing. The #upprev_box div is being generated and the stylesheet is being loaded. If I manually remove the "display:none" property of the div, I can see the div. However, both Chrome and Firefox indicate that upprev.js is not in my list of sources.
Upon further inspection, the problem is that in my custom theme, I deregistered the wordpress default 'jquery' handle to use the copy hosted by google (performance tweak). This plugin hardcodes "jquery" as a wp_enqueue_script dependency. Changing it to "custom-jquery" to match my handle fixed it.
Marcin, consider adding an "advanced option" to either set this dependency manually, or ignore it altogether.