Worth noting though, a lot of the value of GitHub in particular, and to a lesser extent many other services, are extra features like issue trackers, wikis and so on that act to lock you in.
I can certainly see services like wiki and issue tracker hosting being big value-adds with GitHub. Thankfully GitHub does allow you to export your data. (And to be honest, I feel it would be a mistake to use these services if you couldn't export your data.)
The wiki system on GitHub is based out of it's own git repo (https://github.com/[user]/[repo].wiki.git). If you could be bothered to pull/push when changes are made, I'd imagine this content could then be made distributed.
While not distributed/decentralized... GitHub does lets you export your existing issues. The resulting archive can then be imported into services like bitbucket, pivotal, and etc with a little work.
There are solutions for distributed/decentralized issue tracking, but I haven't seen any great way for users to interact with them. (SD and Bugs Everywhere, other users have posted links in their replies.)
