Pushing dynamic content via AJAX and caching the much larger part of the page that isn't dynamic is a great tip -- much better than caching fragments. I will now steal it and appear smart by telling others. :-P
Make sure you run your specific implementation past an SEO who knows what they're doing -- and on this issue, most don't. (The sweet spot is probably caching the whole page for non logged-in users and regenerating that "relatively infrequently", and doing the AJAX trick for your logged-in users only.)