I just built a new plugin that replaces most of the default functionality for Prototype/scriptaculous with jQuery. I still have some work to do completing the rest of the visual effects and writing some plugins for autocomplete and in-place editing but this release is pretty solid.
It includes support for basic AJAX calls (form_to_remote, link_to_remote), RJS, observers (observe_form, observe_field) as well as a limited set of functionality for draggables, droppables, sortables and visual effects.
Please check it out if you are interested. Feedback is appreciated!