I'm soon going to be starting some iPhone Development (3.0) building a simple app that communicates with a web service and uses push notification.
My Objective-C is pretty much non-existent but I've already got some good information from this question and also from Apple's development centre.
Basically if you've developed with the iPhone before what caveats/hints/pitfalls/tips/etc could you recommened in general?
Get "Cocoa Programming for Mac OS X by Aaron Hillegass" and go through the first half of the book. Get "Beginning iPhone Programming" after that and pay the $99 so you can run apps on your phone. There are a lot of design patterns in Objective C that are strange to begin with, but as you work more with them, they make sense.
One of the best things I found in the Hillegass book is his advice in the introduction: "assure yourself that you are not stupid and some things are hard".