Itamar makes an excellent point. That probably is one of the worst problems we have right now. and it definitely needs to be addressed.
Figuring out how code could end up not-in-the-attic sounds like part of the transition plan to me. I do, in fact, think Github means patches are less likely to be abandoned. There's two reasons for that. First of all, you don't have to deal with a patch: you deal with a pull request. That means there's a branch with commits, and it's already in version control. Because that version control lives on Github, it's very easy to pull into your own checkout and work on it.
This lowers the barrier to entry further, and completely gets rid of the distinction between committers and non-committers we currently have (where non-committers don't get to use version control, basically, unless they use bzr or something else, which again is a distinction between workflows for people with and without commit bits).
The main problem I see in this transition is that Github tickets and pull requests are distinct elements in the issue tracker, whereas reviews go on the ticket in Trac/UQDS.