Github has quite a bit of work ahead of them to be on par with say IBM/Rational ClearCase/ClearQuest or CA Software Change Manager (aka Harvest). Definitely wish them luck.
But what's built on top of it are definite wins in comparison to git and Github: UCM and lifecycle integration with ClearQuest.
Most developers don't know how/when to branch. UCM at least sets up a structure and process for that. There's also enough out-of-the-box security to control access to everything that most enterprises would care about. This is supported, documented, etc.
Managing an application's lifecycle is probably far more important in a large organization than most people experience in a small project or startup. Integration with ClearQuest allows everything to be tracked.
Since all of this are essentially "add-ons" to a core versioning tool, there's nothing to say something similar can't be done with git/by Github. It's just that they're not available _now_.
While I don't like Clearcase/clearquest, large companies like them because it allows them to define their own rigid ideas of control within their company. Large companies have a team of people who's job it is to create, manage, and train to that process. Tools like clearcase allow them to create these custom workflows and processes within the company in such a way that they must be follow.
this allow for some pretty complex controls on who can do what, what signoffs must happen for certain code commits, and such.
Git and Github do not offer that fine-grain control that managers of large organizations like. So until you see that type of functionality, don't expect to see Lockheed Martin deploy Github throughout their company.