There was a post about www.cdnjs.com 5 months ago http://news.ycombinator.com/item?id=2132553.
There were some DNS problems at that time and thus most people were concerned about reliability. Now that it is powered by cloudflare.com, do you think that the timing is right to let global javascript hosting to cdnjs.com?
I think that cdnjs is really a great idea and I use it for my new projects. To Google becoming a javascript CDN is far from there core business, I think a distinct entity is much better suited to become the global javascript host.
Using a CDN that you don't control to host your javascript is a bad idea. Don't do it. Not only is it adding another dependency, you are cross-site scripting yourself and probably adding latency to your site due to the added cost of another domain to open http connections to.
CloudFront is cheap, and you can throw all your static files on it.