I can't speak for ad com, but I know that my employer will not give out any information about their employees. I'm talking about big, big hi-tech company. At the best, they may admit that Jane Doe has been working there (or works) there at some point, but they will under no circumstances reveal the title or the salary. Because of the privacy issues, I think that most companies follow this model. I am sure someone on this forum with HR background can shed more light on this topic. That being said, who is stopping you from saying that you have worked for a Blue Chip company as a hot-shot employee?
On the other hand, if you are self-employed, anyone can Google you easily and get more information about you. I am assuming that every self-employed person today has a web site. I would argue that it is easier to verify your background.