I started up an LLC... It's fairly simple and cheap to do and gives you a bit of protection. I bought this book:
http://www.amazon.com/Form-Limited-Liability-Company-CD-Rom/dp/1413307086/ A lot of the info won't apply if you'll be the only owner of the company, but it was still pretty helpful. My accountant was able to answer the couple of tax-related questions I still had. In the end the process was simpler than I expected, but it was nice to have the book to reassure me and point out things that could be problems down the line.
I agree with cpatch about the potential money to be made... It's easy to do some quick math with the Trism numbers and think you could rake in a million a year, but I'm sure the reality is even the Trism guy(s) will have a hard time keeping sales that high for a full year or more. Not to say that it won't be worth it (especially for them!), but my feeling is most people that work hard at it will be making about the same as they'd be making at some other programming job. You might spend less time working on software but you'll spend that time answering support emails, marketing, and other day to day business stuff.
One thing I will say is that the App Store makes it much easier to get started, assuming you have a decent app. It's pretty amazing how relatively simple it is to submit an app and start selling it all around the world. But keeping up that business is still going to fall on you.
So I think the big thing you'll want to think about is just if you really want to run your own business. For me I feel like I barely have a choicemy brain just seems better suited to working this way. For the past several years I've run my own graphic design and web development business, and dabbled in software on the side. My iPhone app will be the first time I've made a significant amount of income from software, and if it goes well I may transition to doing software (web, Mac, and iPhone) as my main business.
Running a business certainly has its ups and downs. For me the big positives are being able to set my own scheduleif I feel like taking a day off I can usually just do itand having the freedom to do the work I want to do. If I want to spend more time writing software I can do that without looking for a new job. The biggest downside is often money. If business is slow it may be tough to pay the bills. (I'm struggling with that a bit right now since I put off my regular design work for a few weeks to work on my iPhone app.) And while it's easy to take random days off now and then, it's hard to take a real vacation any other time than the end of December. With my design work it's inevitable that a client desperately needs something done before I go, with software it's support emails piling up. Since I work at home it can also be tough to just plain stop working at the end of the day or over the weekend.
Overall though, I've found both running my own business and creating iPhone software pretty enjoyable and rewarding so far. I think it's definitely worth considering, and the App Store can be a big help in getting your business of the ground. Just make sure it's what you really want, and make sure you have a planif your software doesn't sell as well as you'd like, you don't want to get stuck unable to pay the bills.
Good luck!