Manual:$wgHandheldForIPhone

Details
Apply "screen" and "handheld" media specifiers for style sheets are transformed such that they apply to the iPhone/iPod Touch Mobile Safari, which doesn't recognize "handheld" but does support media queries on its screen size.

Consider only using this if you have a *really good* handheld stylesheet, as iPhone users won't have any way to disable it and use the "grown-up" styles instead.