This is your good old friend Epocrates in a great mobile format specifically for the iPhone. Download it from the iTunes AppStore. It is free and does everything you need.
This is your good old friend UpToDate in a great mobile format for the iPhone. Type www.uptodate.com into your Safari web browser on your iphone. Click on begin new search, and it will redirect you to a iPhone specific formatting. At that page, click on the "+" sign at the bottom of the window. Then click "Add to Home Screen". This will add a little UpToDate icon onto your iPhone that will take you to the iPhone UpToDate site when you click on it.
DiagnosisPro is an extensive differential diagnosis calculator with both an online version used frequently in attending rounds and a mobile version available for the iPhone as well. Go to http://m.diagnosispro.com on your iphone safari browser and you will be at the customized iPhone interface with diagnosispro. Add that page as a bookmark on your home screen.