People keep bringing that up, but that's missing the bigger picture: GitHub is more than just a git host, it's also a "social hub" where people can fork repos and offer changes back (pull requests), a passable bugtracker (issues), even wiki, and even web host (github.io).
Many people have warned that using these added-value features makes you dependent on GitHub (Linus Torvalds has a kernel mirror on GitHub, but for this reason refuses to use its other features). Migrating all that metadata is hard (is it even possible?).
Joey Hess' github-backup backs up everything github knows about a repository (including branches, tags, other forks, issues, comments, wikis, milestones, pull requests, watchers, and stars) to the repository.
I'm sure most of these could be re-imported into an alternative service through its API, even if with some loss of fidelity.
(for others: he's also the creator of git-annex, an out-of-band file storage extension to git, and for a long time was the maintainer of critical parts of Debian's ecosystem. He likes to live in a yurt: http://joeyh.name/yurt/)
I think Github is a prime example of how much a "social hub" can add. In the 6 years or so before GH, do you think I ever send a patch upstream? Now, collaborating is almost easier than not doing it, and it's an awesome community.
Oh, but they do add a lot of real value. In fact, they're extremely efficient at making this value - so efficient that they don't get to monetize anything but a small fraction of it.
Many people have warned that using these added-value features makes you dependent on GitHub (Linus Torvalds has a kernel mirror on GitHub, but for this reason refuses to use its other features). Migrating all that metadata is hard (is it even possible?).