According to my records, the Apple Stores I'm tracking (near Alpharetta, GA) updated stock at 02:27AM EST today. Stock was available until 02:58AM EST (it cought me asleep, so bummer).
Then stock updated again at 04:15AM EST, with stock showing up until 04:33AM EST, this time I caught the alarm however I wasn't able to grab any units.
And most recently, at 11:16AM EST, stock updated again, which lasted until 11:31AM EST. However, again, I wasn't able to successfully reserve one.
In all cases, I would get all the way up to the check out screen at which point it would fail, and immediately stock would show as unavailable.
Hope this helps.