I followed a howto for adding that hot AJAX loving to a Rails app, and now my gestating bookmark manager has a slick add-a-new-bookmark-without-reloading-the-page form, making it feel pretty responsive. I really wish del.icio.us had implemented that - it takes a LONG time to reload the page (with huge tag cloud) when adding a bookmark...
Anyway, I pieced together code from a howto and a wiki page, and it took me maybe 30 minutes to get it going (with Family Guy playing in the background, so not 100% attention-requiring).
It's not perfect - I have to clean up the new-record-displaying code - but it works pretty darned well. The newly saved bookmark is displayed at the top of the bookmarks list pretty quickly, with no noticeable lag.