Hopefully as jQuery usage grows people come to understand the need for fallbacks.
<script src="//ajax.googleapis.com/ajax/libs/jquery/1.8.0/jquery.min.js"></script>
<script type="text/javascript">
if (typeof jQuery == 'undefined') { document.write(unescape("%3Cscript src='/path_to_scripts/js/libs/jquery-1.8.0.min.js' type='text/javascript'%3E%3C/script%3E"));}
</script>until you modify your own js (not that rare and uncommon) and it's back new and shiny again. from the cdn, it's just already there sitting in the browser's cache.
> Is it really that big of a perf hit
maybe not, but if you had the option to wake up with your teeth already brushed, wouldn't you take it? :D
if(!jQuery)document.write('<script src="jquery.js"></scrip'+'t>');