Here is an easy one. Update the web.config to include iphone in the regular expression used by BlogEngine.Net to determine whether the browser is a mobile device.
<!-- The regex used to identify mobile devices so a different theme can be shown -->
<add key="BlogEngine.MobileDevices" value="(iphone|nokia|sonyericsson|blackberry|samsung|sec\-|windows ce|motorola|mot\-|up.b|midp\-)"/>