That said, I'm not sure what would you like to do in such terminal?
If you're going to do real job, just do in your real terminal emulator, xterm, rxvt, you name it. What's the point of having GitHub terminal in browser? If you want to do quick'n'dirty fixes/improvements directly from the browser, then it's like asking yourself for troubles, because there is quite higher chance that you'll commit a mistake (even literally speaking) than if you'd be using your own terminal that you work with on a daily basis.
You may want to check a few CLI tools:
Git was initially a set of commands from terminal. A lot of programmers use terminal everyday. Even now I have terminal opened for doing different stuff. This terminal-nature of git itself and of github audience makes it obvious it would be used.
The other interesting question is how it would be implemented. With my repos on github (as opened as private) I think it would be great to have an opportunity to do some set of things from the site CLI.
P.S. I understand there are a lot of those who prefer GUI over CLI but I believe I am not the only one who wants to have CLI also available.