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Jesper has some good points about the loose ends Apple has yet to answer.

Daniel Jalkut asks Apple to Fix the Sandbox, which isn't really Mountain Lion specific, but is still looming ((Even with the pushed-back - again - deadline, it's looming.)). I'm not keen on his suggestions, but they started a discussion so they're valuable for that alone.

Of course, this post from Wil Shipley is still worth a read (or a re-read).

Macworld has a Hands On with Gatekeeper.

Tidbits has some more details on how Gatekeeper works. Apparently you will be able to install unsigned software via allowing it in a contextual menu.