Pulled my hair for a couple of hours trying to fix this.

I have a completely normal div to which I apply

$("#theDiv").draggable({ "helper": "clone", "revert": "invalid", "zIndex": 5000 });

After the first drop, be it on a droppable or in an invalid place, the div won't be draggable again.

This happens with jQuery 1.5. I exchanged it with a 1.4.2 jQuery and it works perfectly.

I'm seeing in the combo that there's a new 1.5.1 version. I'll give it a try with that.