UPDATE
The bookmarklet hack below is broken due to XHR issues and API changes.
Thankfully Github now has "A Whole New Code Search" which does the job superbly.
Checkout this voodoo: Github code search userscript.
Follow the directions there, or if you hate bloating your browser with scripts and extensions, use my bookmarkified bundle of the userscript:
javascript:(function(){var s='https://raw.githubusercontent.com/skratchdot/github-enhancement-suite/master/build/github-enhancement-suite.user.js',t='text/javascript',d=document,n=navigator,e;(e=d.createElement('script')).src=s;e.type=t;d.getElementsByTagName('head')[0].appendChild(e)})();doIt('');void('');
Save the source above as the URL of a new bookmark. Browse to any Github repo, click the bookmark, and bam: in-page, ajaxified code search.
CAVEAT Github must index a repo before you can search it.
Abracadabra...
Here's a sample search from the annotated ECMAScript 5.1 specification repository: