See memmel's explanation downthread. Also, note that no matter what the temperature, all liquids have a vapor pressure of some sort - that's why they evaporate. This vapor pressure increases with increasing temperature. When you get to the boiling point, this vapor pressure equals atmospheric pressure and the liquid vaporizes off at a rate determined by how fast the heat is flowing in to the liquid. When you blend liquids, the individual vapor pressure of each component liquid is protated by that component's mole fraction concentration in the liquid. So, while pure butane's vapor pressure is above atmospheric pressure at room temperature, butane in solution with gasoline has a vapor pressure much lower, so you don't see bubbles of butane boiling out of the gasoline. It does evaporate out though.

FYI, pure butane has a boiling point of approx. 0 C or 32 F, IIRC, so that's why it's not used for heat and cooking like propane is. I think propane boils somewhere near -40 C / -40 F (where the two temperature scales cross).