Maybe Safari on the iPhone prefers the stricter xhtml rules which are case sensitive in that the html tags have to be in lower case. Just because they work in the other browsers doesn't mean it's right.
According to Apple's iPhone dev web site it supports the following;
- HTML 4.01
- XHTML 1.0
- CSS 2.1 and partial CSS3
- ECMAScript 3 (JavaScript)
- W3C DOM Level 2
- AJAX technologies, including XMLHTTPRequest
Are you at least declaring the html version in your headers? That can make a difference with the more picky browsers. (almost everything except IE for the most part). This is my guess. I'll check into it more. In the past few years I have all but transitioned to xhtml (at least in transitional mode) so I haven't noticed any problems so far with the sites I develop.