I have a website where we use a technique to speed up loading times based in a combination of AJAX + caching. Basically, when we have a section in a page with content which is slow to retrieve, we first look if it's cached. If it is, then we serve the content, if it's not, we serve a placeholder and then make an AJAX call in the client to retrieve the content, wich is now cached for subsequent requests.

As a consecuence, sometimes you get the entire page content in the first request, and sometimes you get those placeholders, wich get filled inmediatly with the responses of the AJAX request.

You can see an example in the results count by category in the right column of this page:

http://www.inzoco.com/crits/2-1-3-28-185-0-28079-0-0/listado-piso-en-alquiler-en-madrid-madrid.aspx

I'm worried if it could be seen as cloaking by search engines because if you make a request for a page wich content isn't cached and then ask again for the same page, you would get different responses, the first with the placeholders and AJAX requests and the second one with al the content rendered.