When NOT to use AJAX in web application development?

I'm building a web application with the Zend Framework. I have wanted to include some AJAX type forms and modal boxes, but I also want my application to be as accessible as possible. I want my application to be enhanced by AJAX, but also fully functional without AJAX.

So as a general guideline...when should I not use AJAX? I mean, should I bother making my application usable without AJAX? Or does everyone have AJAX enabled browsers these days?