My understanding is that Mountain Lion will be released in a configuration that forbids the installation of non-signed applications by default through Gatekeeper.
I currently distribute an open source application; this is a hobby project and I do not charge any money for it. Am I able to sign the code for this application without purchasing a Mac developer membership, or is Apple requiring that all developers pay $99/year to release applications that install on the default configuration of Mountain Lion?
Note that “you can manually override Gatekeeper by Control-clicking the app and choosing to open it.” Not obvious from Apple’s description is that you only need to do this once for any given app, so it shouldn’t cause much inconvenience to your users one way or the other. Apple seems to understand that if Gatekeeper is inconvenient, people will simply turn it off.
As for whether only paid program members will be eligible for Developer ID certificates after the release of Mountain Lion, I’m not aware of any statement by Apple one way or the other. Developer ID links currently seem to be redirecting free users to the paid program enrollment page, however.