Are you sure its not antifreeze? The only way i can think of gas getting into the oil is if somehow in one of your cylinders it manages to seep past the rings and not get ignited but wouldnt that make your car run like crap. Its a longshot and i dont even think it can happen unless you accidently put gasoline or somekind of additive in you oil by mistake.
Vegata is correct. The gas is passing past your piston rings. It's normal to have some gas in your oil as a result of it passing the rings. It's just a matter of how much before you get concerned. Do you have forced induction? Run 93 Octane? Run rich?