In the worst case, several things can happen:
- You could be sued, but that is very unlikely as a first step.
- You can receive a cease and desit letter. At least here in Germany this can be very costly. You have to pay the issuer's attorney's fees which can be a couple thousand euros. I don't know how this works in other countries, but most have a similar system.
- You can be forced to "buy" a license for the patent (either by coercing you into a settlement, or directly if the legal system allows it).
What makes this very complicated is that several legal systems apply: The one of your country, presumely American, the one of where Sony resides / chooses to file suit in (USA, Japan, who knows?) and possibly the ones of countries you sell your app in, if you violate any patents held there. It's basically impossible to protect yourself 100%.
What you can do is register an LLC or similar, as Victor said. If you can afford it, you should also try to talk to a lawyer, although I realize this is overkill for most small developers (I certainly wouldn't be able to affort it, and wouldn't want to blow my meager app earnings on one).
One thing you should definately look into is to get legal expenses ensurance. Again, I only know the situation in Germany, where it is pretty affordable, and they'll pay all your attorney fees, but I something similar exists in other countries, as well.
But as Morons said, don't let the possibility of patent violation deter you. The chance of getting into trouble is very small and nobody wants to waste time and money chasing "small fish". The realistic worst case is that they'll ask you to take down your app.
(Oh, and I'm not a lawyer and everything I said is potentially wrong, so don't rely on it.)