Yes, if you load in all your content through AJAX/AJAH or some other form of client side process, google won't get that content when crawling your page. The best way to see what search engines will see is to load the page in a browser and then use "View Source".
There are ways to help search engines find your dynamic content, however, which typically involve creating static versions of the content in addition to your more user-friendly AJAX versions. Google outlines some methods for doing this using specially formatted hash tags here: http://code.google.com/web/ajaxcrawling/index.html
If this gets popular enough, maybe we should standardize a new <link> element to point to the JSON data file that search engines should index. Maybe even a standard schema for it!
If I'm pulling the content in as json and using jquery to place it on the page. Will Google understand this or see my pages as content-less?