We have no plans to do so.
mySociety makes heavy use of GitHub across all its projects, so it wouldn't be just Alaveteli, but many other projects that would need to be considered.
While there are other options, they'll all have their own issues, and we don't have the resources or capacity to host our own equivalent.
Happy for this discussion to take place for those interested, but with the expectation that there won't be a change of position any time soon. We've got to pick our battles, and this isn't where we can best make a difference.