Many popular JS/CSS frameworks are offered via Google's Libraries API (jQuery, Dojo, MooTools etc.). Yahoo also hosts it's own YUI toolkit, as do many others.
Do any high volume/traffic sites actually rely on these externally-hosted resources (without hosting their own copies)? It seems like a great service to leverage, although in my experience I've often encountered these libraries packaged along with the projects I've worked on.
What's the most common practice here? Moreover, is it safe and reliable (based on experience) to use these externally?