Are AJAX sites crawlable by search engines?
Following this guide from Google, AJAX sites may be made crawlable:
http://code.google.com/intl/sv-SE/web/ajaxcrawling/docs/getting-started.html
Search engines could run the JavaScript needed to index Ajax content, but it would be difficult and computationally expensive — I'm not aware of any that actually do.
A well written site will, if it uses Ajax, use it according to the principles of progressive enhancement. Any key functionality will still be available without needing to run the JavaScript.
On the other hand, sites which reinvent frames (and don't use progressive enhancement) using JavaScript will suffer from all the usual problems of frames, but trade orphan pages for search engine invisibility.
I have NoScript installed and active. Both links show the same content (+/- the google header bar). Therefore, the Google cache shows only what is statically there.