I've got a form that uses the jquery autocomplete UI plugin, http://jqueryui.com/demos/autocomplete/, which all works sweet except when you press the enter key to select an item in the autocomplete list, it submits the form.
I'm using this in a .NET webforms site, so there may be javascript handling associated with the form that .NET is injecting that is overriding the jQuery stuff (I'm speculating here).