Mobilejoomla checks for a device user agent, you can change it from your browser or add ?device=iphone to the test url.
Direct from the testphone:
Does TestiPhone set the User agent of the iPhone on request? Unfortunately some apps require that you are using an iPhone by checking the user-agent. You can easily switch your user agent string. The settings for the iPhone are: Description: iPhone, User Agent: Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en), App Name: AppleWebKit/420+ (KHTML, like Gecko), App Version: Version/3.0, Platform: Mobile/1A542a Safari/419.3.