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This is a great article that covers how Apple plans to keep malware off Mac's.

Do you think more app developers will move away from the App store or will they really have no choice if they really want to sell their product?
You can still install apps that don't come from the App Store, so developers aren't forced to use the App Store. Ultimately, the responsibility to keep a Mac malware-free rests with the user, who can avoid all Mac OS X malware that exists in the wild by practicing safe computing.

Mac Virus/Malware FAQ
 
It doesn't look as though sandboxing and Gatekeeper will prevent a determined attacker nor an uninformed user.
 
Yes, you're right that sand boxing will prevent some attacks or make them more difficult.
 
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