In my previous post I said that Mac signing had been turned on, and that it would stay on. Unfortunately, the following morning we caught some issues that only came up after updating. It took a bit of time to resolve those, but as of this morning we got them worked out, and signed mac builds are back. There are a couple of follow-up issues to address, but we're in a good enough state to backport signing to Aurora and Beta, and ship it in Firefox 13.0. On Saturday evening I'll be enabling it on Aurora, and on Monday morning on Beta - assuming no new issues come up.
If you see ANY issues related to updating on Mac please file a bug and cc me (:bhearsum). It's very important that any issues are brought up immediately, as we intend to ship this to the release channel on June 5th.
Huge thanks (again) to Steven Michaud, Ted Mielczarek, Erick Dransch, and anyone else that helped make this happen.