That's one of the reasons why I wouldn't move to the US and start a business. I think a software company really have only two choices. 1. Ignore patents completely and hope for the best. or 2. Have trouble sleeping, knowing that you might have to spend a large amount of money on a lawsuit that you will probably win anyway.
IANAL but i think non-US startups are also targets for patent lawsuits if they have US users. And many startups have little chance of success if they have to stop serving US users.
BTW, I live in Brazil where software patents are not considered valid.