I think we will only see a github for mechanical design once there is a good OS alternative to solidworks and it's brethren, freecad (http://www.freecadweb.org/) is in the right area but you can't underestimate how huge these packages are. I think it will be another 10 years to see something like freecad reach the level of the commercial software.

Part of the issue is that there is not open standard for an editable parametric cad file. All the open file formats are 'flat', its like flattening a photo shop psd into a bmp.

Anyone collaborating on an open platform will at the moment still need the same expensive software to edit the files. That is the barrier to entry, not the availability of a github for CAD.