A very interesting article has been posted over at SoftwareDeveloper.com about using AJAX so that your site isn't hiding content from the likes of Google : How to : Get Google and AJAX to play nicely.
The article gives some good examples of what to avoid doing and alternative methods to those generally used that will probably be more efficient. AJAX is a great addition to have, just make sure that you are not blinding the search engines from reading your content when you use it !