Stevenf from Panic has a great write-up on Gatekeeper and code signing in OSX Mountain Lion. Panic is a leading third-party, independent software developer for OSX. They make Coda, Transmit, and a few other well-regarded apps. The majority of their apps are not available through the App Store, so they're quite sensitive to the needs of those who wish to independently distribute software.
Overall, he's extremely positive about the changes in OSX Mountain Lion. He does mention one important caveat about feature parity between App Store and non-App Store software. (I agree with his concerns there)