Developing for iPhone OS can be overwhelming and quite difficult to know where to start. For people with little experience with Objective C like me, it can get really tricky. That's why I love the list that webdesignerdepot.com has put together. It seems many other like it as well, and I'm not surprised.
You probably know the official iPhone Dev Center, where there are loads of resources including videos. You might want to check the Stanford lessons on iTunes downloadable for free.
Whatever your aspirations are, check this extensive lists of links to tools and tutorials as a starting point. It has been very useful for me and I have bookmarked it in my favourites.