Developer ID is the name Apple's giving their "trusted developer" system for deploying safe and tamper-secure non-Mac App Store apps to OS X. OS X Mountain Lion includes the option, along with Mac App Store only and all apps, regardless of source, as part of their Gatekeeper feature.
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.
While this is Mac-only for now, and I'm hard pressed to imagine Apple bringing it to iOS any time soon, it's an interesting approach that allows for a lot more flexibility for developers, variety for users, and security for the platform all at the same time.
While an official option to sideload apps onto the iPhone, iPod touch, and iPad would no doubt be popular with those who jailbreak, there don't seem to be a lot of apps on Cydia, the jailbreak store, that Apple would allow unto iOS even via Sideload. For example, anything that modifies the core OS system would likely still be prohibited, making the entire concept pointless on iOS.
Can anyone think of a likely use case, or examples of iOS apps, that Apple would actually allow to be sideloaded but not allow in the App Store?