http://code.google.com/apis/libraries/devguide.html states that you need to sign up for an API to use the CDN. But why? You can clearly use it without.
Doesn’t linking to google.com/blah/jquery.js?myAPIcode mean that a new version of jquery.js would be downloaded, rather than using a cached version? So using an API would slow down my website, and not using one would speed it up? Is there any advantage of using an API? Or is it just so Google can “contact you directly if we detect an issue with your site” AKA track you?
Given the http://html5boilerplate.com/ guys don’t use an API, I think it’s best not to. No one seems to know of a good reason. And Google tracking your site isn’t a good reason (you probably use google analytics anyway!).