[TimTheTinker]

For the most part, yes, unless you're a non-practicing entity (patent troll).

Large companies carry massive war chests of patents, such that anyone who makes software is most likely violating several of them without knowing it. They defend themselves from patent lawsuits with countersuits from their war chests. (Trolls are immune, though, because they don't make software and therefore aren't infringing in the first place.)