Since last week, I've had issues with draggables suddenly 'freezing' on the facebook canvas page, for no apprent reason.
This was tracked back to improper z-indexing for certain elements. So there.
I have also had a request out from a developer who wanted a special kind of snap-back, so that if a draggable is dropped anywhere on the canvas, it will snap back to its original position - unless it's dropped one a specific dropzone. That works now, like a clockwork :)
And the off-by-two, works on sundays, et.c. fetaures of the snapback are gone too. Now you can snap back relatively positioned elements with no worries.
The zip-file with the sample facebook application included (and a README!! Gosh!) is here; http://supercodex.com/backface/backface.zip
And if you just want to good around with the demo, you can find it here; http://apps.facebook.com/backface
No need to add or anything, just go there, no worries :)