Not completely sure on this one, I don't even own an iPhone (yet), but AJAX is a framework of different techniques used all over, and AJAX just brought them together.
Ajax is a vague term, but mostly it means:
* XHTML and CSS for presentation
* the Document Object Model for dynamic display of and interaction with data
* XML and XSLT for the interchange and manipulation of data, respectively
* the XMLHttpRequest object for asynchronous communication
* JavaScript to bring these technologies together
(source: wikipedia)
So, I'm sure that iPhone supports XHTML & CSS, XML and JavaScript. What I'm not sure of, is if XMLHttpRequest object is present, but if the iPhone version of safari even closely resembles its big sister, I'm sure it's there.