Name: general.useragent.override
Value: Mozilla/5.0 (iPhone; U; CPU like Mac OS X; en) AppleWebKit/420+ (KHTML, like Gecko) Version/3.0 Mobile/1A538a Safari/419
To go back the original value, you just have to right-click on the item and select "Reset".
While the iPhone versions usually have a better user interface, in some cases they also have more features (Mobile Google Docs lets you view presentations only on the iPhone). I wonder how they will look on a Google Phone.
Google Reader (compare it with the standard mobile version)
{ Thanks, Martin Porcheron. }