After calling that, you can create a view (index.iphone.cshtml) and it'll be automatically picked up when the condition's matched. This example uses the user agent, but you can use any logic you'd like - cookie / database call / time of day / whatever.
Very nice! What's the best way to develop and debug mobile sites locally? Is there a quick way to force either my browser or VS to go into 'mobile' mode?
The small improvements to Razor (e.g. automatic ~/ parsing and conditional shortcuts) are also nice time/trouble savers.