Unfortunately, the software they have for doing this runs in MIT Scheme, and I own a PowerBook running OS X. It's not really a problem to SSH into a computer which runs their system, but it's nicer to have access to it on my own computer. So: I've coded up a bare-bones version of the scmutils/differential-geometry system for myself in PLT Scheme. It does no symbolic manipulation (only numerical calculations allowed), and doesn't have much of the nifty stuff that comes with scmutils proper, but it is able to compute on arbitrary manifolds and to handle Lie groups. I've been using SchemeUnit to run tests on the code as I go along, so there's a pretty good chance that it doesn't have major bugs.
I've posted my current darcs repository here; a darcs get http://web.mit.edu/farr/www/SchemeCode should get it for you, if you're interested. I haven't really written any documentation yet, but the test files should give some examples of how the functions are meant to be used. Comments are, of course, welcome at email@example.com.
I'm not really sure why I'm posting this for the wider world right now (since it's so incomplete and "internal"). I hope someone finds it useful or interesting.