I got this the first time I visited google forums. From there when I clicked on News, Gmail, etc. in the header I get the iphone interface.

You don't have to make any useragent mods. Here is the url - http://www.google.com/news/i?hl=en&source=mog&gl=us

From here, you could navigate to "more" on the header to find other google services with similar interface though google voice still renders in the non-iphone interface.

I guess google does some js based checks whether the browser will support the UI. If so, then it renders otherwise it renders the default UI for the useragent.