Scott! Meanwhile, I have encountered a similar problem to #5280 with the 'wobbly' behaviour; this was caused by setOffset in offset.js of core using parseInt instead of parseFloat (to explain: ui-autocomplete would write fractions into the CSS which were then parsed back if autocomplete was executed a second time, thereby causing the 'wobbling' effect). Using parseFloat in setOffset seems like the best fix to this problem, however, as I understand it, this would make the workaround of #5280 and #6000 superflous, or in the worst case even buggy.
I have tried to remove the two lines completely (while also using parseFloat in core) and have not encountered the problem since. If core decides to use parseFloat instead of parseInt I would suggest removing the two lines from ui-position entirely.
Also see http://bugs.jquery.com/ticket/7730