Using html5shiv/shim for IE and HTML5 elements? Use a local copy.
A long post here explains issues with linking to an external source for 3rd-party JS, but especially for linking to the HTML5shiv that allows IE8-and-under to understand HTML5 elements such as <nav>.
The author is specifically mentioning the html5shiv stored on google.
-it's not minified/optimised
-it's out of date
-it won't cache (so everyone's re-downloading it every request)
-it won't offer partial requests
-it's not code you control.
And there are interesting comments after the article.