The Safari web browser on the iPhone always shows itself as a mobile web browser (based on webkit). There is no built in function inside the safari that lets you change the user agent inside the General or Safari’s settings.
However, with a cydia tweak called "user agent faker", you can easily change the user agent to anything you want. Of course, to install it, you will have to jailbreak your iPhone and install Cydia as well. It is available from the big boss repo (which is installed in Cydia by default). Just search for ‘user agent faker’ inside Cydia to install it.
Also check out – how to mark all mail read in iPhoneSearch queries: User Agent Strings iphone jailbreaked