Direct iPhone visitors to different stylesheet?

There is a global $is_iphone variable, which you can check against in your code.

global $is_iphone;
if ( $is_iphone ) {
   // do something if $is_iphone is true
}

In the realm of your style.css however, I think @media queries may be easier. style.css is hard-coded as a required stylesheet, and there are no filters which can alter what the default stylesheet name is.

You can definitely use it for secondary stylesheets, however.

function my_iphone_styles() {
    global $is_iphone;
    if ( $is_iphone ) {
      wp_enqueue_style( 'my-iphone-styles', get_stylesheet_directory_uri() . '/css/iphone.css' );
    }
}
add_action( 'wp_enqueue_scripts', 'my_iphone_styles' );