This fix introduces a problem, at least in our application. If an item is moved from a list of draggables into a sortable, and its receipt is processed using .sortable('serialize'), the serialized list will contain an empty string where the item's id should be, and thus it will not get processed. In our case, if an item is added to the sortable, sortable calls an external routine to store the new object and sort order in a database and then repopulates the sortable with the data. Upon updating to jquery-ui 1.8.13 with this fix, items dragged to the sortable appear, but the item's ID does not show up in the serialized data, so it doesn't get added to the database and disappears when the pane refreshes.
I'm not too sure about whether or not this bug is a bug--it seems that authors using jquery-ui should take steps to make sure that when their action is complete, there isn't a duplicate id. For example, if an item is dragged from one list to another, it should be removed from the original list.