One of the main reasons I own an iPhone is to have access to a map when I'm out and about. The iPhone comes standard with Google Maps, but I find its always good to have a backup map. (Google Maps isn't perfect, but you didn't hear that from me.)
That's why it was good to hear that MapQuest now has an optimized version of its site for the iPhone. Even though the iPhone lets you surf the web, surfing normal pages is still a pain. Whenever a site has versions for mobile devices, it's so much better for the mobile user.