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Developers are getting this email today from Apple:

The Mac App Store is the safest place for users to get software for their Mac, but we also want to protect users when they download applications from other places. Developer ID is a new way to help prevent users from installing malware on their Mac. Along with Gatekeeper, a new feature in Mountain Lion, signing applications with your Developer ID certificate provides users with the confidence that your application is not known malware and has not been tampered with.
Get your applications ready for Gatekeeper today. It’s easy to get started with Developer ID using the automated certificate request tools in Xcode 4.3 or the Developer Certificate Utility.

For developers that don’t want to distribute their Mac applications on the Mac App Store, Apple offers a new way to make sure Apps are from trusted sources. With a Developer ID and Apple’s new Gatekeeper, users (and IT administrators/parents/kids/ etc) can be sure that only applications signed by Apple developers are able to be executed on a Mac.

Thanks Jonicious!