If I were in the US, I would suggest to register an LLC business. That way, if you go bankrupt, they can not go after your personal assets.
Oh man prepared to get OWNED by tax like they have done me. That totally killed my motivation to work since self employed users have to pay so much tax. They pretty much take half of my profits now and not to mention the trouble of you go through keeping track of every penny you spend and reporting it.
General piece of advice for all UK earners here - speak to a chartered accountant if you haven't already. DP members are very clued up but I doubt there is anyone here who is qualified to give you the correct information. An accountant will give a professional answer to all of your questions as well as good advice about your tax standings and how to maximize what you take home. This includes when you should register an LLC. If you're concerned about the expense of an accountant and you're turning over a decent amount of money then it's probably costing you more *not* to have one. One chap who comes highly recommended for small businesses and internet based business is James Smith.