@NogDogJul 21.2009 — #AJAX is just a specific use of JavaScript. Any browser that supports reasonably current versions of JavaScript will support AJAX (i.e. pretty much any 21st century browser).
Of course, any user who has disabled JavaScript (intentionally or unitentionally) will not be able to use AJAX. Also, some hand-held devices may be more likely to have it disabled (or possibly not available at all). So you need to decide the likelihood of users not having JavaScript available/enabled, and if you decide to use AJAX whether you need to also include "fall back" code for dealing with non-JS-enabled users.