When Apple announced that the "sweet" iPhone Software Development Kit was going to be AJAX and "Web 2.0 stuff", most developers scoffed. A few went to work, and now we are starting to see some web applications built specifically for the iPhone.
First up is a very nicely designed shopping list application named OneTrip. It was the first iPhone application released as far as I can tell. It's also my personal favorite.
Then 37Signals Ta Da List web app simply senses if you are hitting their website from an iPhone and adjusts to look like this:
Notice how these iPhone web apps follow or try to follow very closely the iPhone look and feel. This is a testament to how these developers really get the "build the experience" ethos. That said, even with all this focus on looking "iPhone-like" without the smooth transitions, it's really quite a different feel.
An attempt at a full list of all iPhone web apps can be found at iPhone Application List, but be prepared for some ugly looking iPhone apps. I think the delay in releaseing a "real" iPhone SDK may actually help cement the iPhone experience. With a solid customer expectation of what it means to be an "iPhone app" this could actually lead to an better user experiece with all 3rd party iPhone applications long term. Let's hope!
Apple's official documentation and guidelines for developing for the iPhone are located here: http://developer.apple.com/iphone/