Well (as another Federal coder), one problem is the unfunded mandate issue. If this comes with resources to maintain public repos and updates, sure. Otherwise, if someone emails me I share the latest snapshot but only as my time and duties permit.
I realize that setting up an automatic integration between your existing source control system and github might be nontrivial (eg if you are using an unpopular source control system for which no easy conversion to git) so this won't work for everyone without funding (and it would be nice if some were made available for that)
But if you already use git (or something like SVN or hg which can easily interoperate with git) or don't currently use any SCM (!) then just publishing to github is a nobrainer.
Am I missing anything?
Of course, the last one is the worst. A lot of groups are getting their act together when it comes to starting open-source, but it is met with a lot of skepticism.