I spoke today to Jon Placa @ ProtoExchange about how we're using Github on The Public Radio. In a followup email, he asked: "Is there any reason why you aren't using Github to host your actual design files (beyond gerbers)?" My reply:

No singular reason. But...

Only I work on our mechanical design, so there's no need to collaborate.
When I was collaborating on mechanical design (last job), I used Autodesk Vault - which is actually pretty good.
Mechanical design is even more fragmented - it's not as if me posting .ipt and .iam files on Github means they're really useful to anyone else, because maybe they're on Solidworks or an earlier version of Inventor - or, more likely still, on some consumer grade system.

That said, the original goal was to be open with all of it, and I would like to post those files there too - if only for posterity's sake.