Novo Digital Camera have created a custom housing for GoPro 3 that lets you fit C-Mount lenses and even disable auto exposure using a custom CPU!
Pretty cool huh?
Apparently it's pretty complex (and thus we can assume expensive to make) so it won't be a product to buy, but they will be hiring them out.
Just goes to show how far you can go with a mod if you have the expertise...
More info here: http://www.viewfactor.net/blog/?p=331