It would depend highly on how his business was structured. For example, the company I work for has been through two different structures. First we were just owned by my boss, and his taxes were tied to the company. Now we are an S Corp, which means the comapny and the person who Owens it are different legal entities, and thus file taxes seperately. If Joe the Plumber has a large sub contracting business and doesn't want to risk losing his house over what his dumb employees do, he should probably incorporate.