updated 03:13 pm EDT, Tue August 19, 2014
Re-signing mandatory for existing apps
Despite recent claims, a Dev Center security breach may not be why developers are being asked to re-sign Mac apps using OS X Mavericks, sources say. An alternative reason for the switch hasn't been mentioned, but unnamed sources are countering reports yesterday from other unnamed sources. In the earlier rumors, it was claimed that one or more hackers had managed to obtain not only Gatekeeper keys but "virtually every key Apple used for everything."
The contradictory report that claims that everything is fine and no security breach has taken place is from 9to5Mac, but offers no proof of "confirmation" and even fewer details on its sources than the original report, which came from TUAW. Given that all the sources involved are anonymous, it is difficult to know which version to believe -- however, Apple's requirement to change and re-sign approved applications does seem to fit the explanation of a security issue better than the complete lack of alternative explanation for the move provided by the counter-claim sources.
Gatekeeper is intended to protect OS X users by blocking apps from developers who are not registered with Apple. While Gatekeeper can be set to only allow apps from the Mac App Store to run, it is generally set by default to allow both Mac App Store apps and apps from any registered developers to be installed and run without restriction, even if they are not available from the MAS.
Users who want to run an app that is unsigned can do so if they wish, but it requires an extra step that is not widely known (right-clicking to use the contextual menu to open the app instead, on first run only) -- which means that developers skipping Gatekeeper are likely to lose business. There are legitimate incentives to ignore that though, such as the $99 per year cost of registering as a developer. Some legitimate developers - such as Joel Barriere, maker of the well-regarded utility OnyX - cannot justify the cost of registering, since his only product is freeware.