I can make some development philosophy suggestions, based on my experience. For the technical issues of getting started, you can search around here or in Apple's excellent documentation.

Make your first application free. I decided to do that, and it's worked out very well. A free project is a great way to become acclimated to the frameworks, tools, submission process, etc. Good free applications can also get you a lot of exposure and build goodwill. For example, my application ^[1] has been downloaded over 340,000 times worldwide. Hopefully, I've made a favorable impression on many of those users, who may be more likely to purchase something I write in the future. Even better, open source your program and allow others to learn from your work. You'll get a tremendous number of hits to your blog or site just from other developers looking for code examples.

You asked for how to have fun, well I can tell you that I've greatly enjoyed hearing from educators and scientists around the globe. By releasing the code for the application, I've had a chance to talk to a number of great Mac / iPhone developers and learn from them. I've had more fun doing Cocoa development on the Mac and iPhone than on any other platform I've worked on.

The next thing I'd encourage is for you to challenge yourself in the development of your application. If you can write it in under two weeks, don't bother. Almost all of the low-hanging fruit has been plucked in the App Store, with decent free versions of most ideas. A talented developer can differentiate himself from the crowd by writing an application that's difficult to do a good free implementation of. In particular, I'd love to see more applications aimed at scientists, engineers, and even students.

If you're worried about the limited exposure that a higher-priced, quality application gets now, I don't think that will be as much of an issue in the future. Significant improvement have been made to the App Store since launch, and I believe even bigger ones are to come that will make it much easier for users to pick out the quality applications from the garbage. Hopefully, the market will sort out in the way that it has on the Mac, where the polished, easy-to-use applications become the most popular.

From a technical perspective, start at http://developer.apple.com/iphone/, where Apple provides a huge amount of free info, including documentation, sample code, etc. If you haven't done Objective-C programming, consider Aaron Hillegass's book "Cocoa Programming for Mac OS X". Although it's written for Macs, most of it applies.

You will need to get a Mac, if you don't have one. That might seem obvious, but many people don't seem to realize it.

On non-technical issues, don't make the mistake of thinking that getting into the iPhone app store is an easy road to "a ton of exposure". The app store is already crowded, and just being listed there doesn't do much to promote an application. It's a convenient way to process orders but it's also extremely easy to be lost in the crowd if you don't take the time to market the software in other ways. Apple doesn't market your app for you, they provide a place for people to buy it. If I look through the iPhone apps I actually use on a regular basis, all of them are apps that I've heard about through other sources. I've tried just going to the store and searching but I haven't found it to be an effective approach.

It's also gotten quite competitive, to the point where apps not in the top listings often end up in a race to the bottom on pricing, hoping that dropping the price again and again will bring in new customers (see "How to Price Your iPhone App out of Existence" ^[1]). There have been numerous stories about the fortunes made by a few iPhone developers, but they're the exception rather than the rule.

I focus on this aspect because it seems there are a lot of people who do think that getting into the app store is the end of their marketing efforts. In the first couple of months after the store opened this may have been enough, as apps could ride a wave of excitement that apps were finally available. That's mostly petered out now.

Simply putting your apps in the App Store is not enough. There are thousands of apps there vying for attention. Users are reluctant to spend more than 99 cents on any app, so you'll probably need a lot of volume if you want to make a living at it.

You really need to be prepared to do some marketing outside the App Store if you want to make money without relying on pure luck.

Newsweek has an article about some of the lucky iPhone developers: http://www.newsweek.com/id/174266

FWIW, my 99-cent iPhone app has sold about 200 copies in the past three months. I'm not going to be quitting my day job any time soon.

On the technical side, I have found the pragmatic programmer's book http://pragprog.com/titles/amiphd/iphone-sdk-development, together with the sample code at the apple site, very useful.

I managed to get a working app together with just this, despite no prior Mac, Cocoa or objective-C experience. I did come from a pretty good background in Java and C/C++ though.

Without a decent book and some good examples to start from and adapt, it is a struggle though. Even with this, be prepared to spend a few days tearing your hair out over some bug. Of course, SO is another good resource to ask questions if you get really stuck.

As other answers have pointed out, you'll need to sign up with the apple dev program, and get a mac. A mac mini or an entry level macbook is fine, but make sure you get at least 2G RAM. And, it may be pointing out the obvious, but having an iPhone and/or iPod touch to develop on helps. :-)

You'll also need to figure out the code signing that the iPhone requires, both for app store submission and development itself...but I was able to get this going just by following the apple dev site instructions. You really need to RTFM on this one, because it is far from intuitive and a bit flaky.

Ideally you need to get a 'buzz' around your app even before you release it, by starting to market when you start to develop. The marketing is just as essential to the development and should follow along side it. You can't assume that sales will just pick up from nothing from when it is approved and goes on sale. If nobody knows about your app, then no matter how good it is, it will never make it big, unless you are very very lucky!