Really? It's easy: Add an additional service that doesn't store its data in git repositories and doesn't allow for that data to be exported easily. Then make it enticing by integrating it really well with the stuff you're already using. Now you're going to think twice about migrating away, because suddenly it's hard.
Not like it's rocket science, and it happens all the time when companies find themselves in a corner (obviously we're talking a hypothetic scenario here). Heck, it even happens with initial good intentions on all sides, because a well-integrated add-on may be a lovely thing by itself.
I really don't want to be panicmongering, but if you "can't really see a way" you're not cynical enough. Don't stop thinking at "it's a DSCM, we'll be safe". You're not using git, you're using GitHub. That's why you're there, right?
How do you migrate this data? There's ALREADY some lock-in.
FWIW, I was involved with putting together the git hosting infrastructure used by the KDE community (which I wrote about at length here: http://news.ycombinator.com/item?id=2972107), which incidentally was done only after negotiations with Shortcut AS - then the company behind GitHub competitor Gitorious.org - fell through, since we couldn't come to an agreement acceptable to both sides. Things like data export guarantees were part of our goals back then.
And there's more data you want to be able to export than you might think, too. For example, if you actually let accounts push directly to your repository, you very likely want to have logs of what account was used to push what data for liability reasons - that's data that's not in the repo, because push != commit. Rather, it's data coming from the infrastructure that handles authentication for you.
See comments in this thread "I find myself frustrated any time I run into an open source project whose git repo is not on github." and elsewhere.
Many developers at work don't know how, don't want to learn, and don't want to use any system that's not github.
Github has such a huge first to market advantage, it is/will be extremely hard for better solutions to ever get noticed.
paypal, ebay, (in some peoples opinion gmail and gsearch) all demonstrate this same effect.
Huh? What on earth do you mean? You mean that they can't figure out that instead of going to the GitHub repo, copy and pasting into `git clone something.github.com` they need to clone a different URL? Sounds like your fellow employees are... uh... not that stellar.
And yes incumbents fall, it just takes way more than Only being better.
http://steveblank.com/2010/10/04/why-pioneers-are-the-ones-w...
I have always seen this as a testament to github's quality - if the switching cost to your users is a couple of shell commands, you better make sure you offer a good product!
I also keep my projects in a local gitosis repository, mainly so I'm not relying on github as part of my deploy flow.