Another +1 for this. In fact, when drafting a semi, you actually don't need to pedal that hard. You are being pulled into the strong vacuum created behind the truck. So when doing 50 mph, a cyclist doesn't actually need to have a cadence and gear that equates to that speed. You will find you don't even need to pedal at all for brief periods. The faster you go though, the more you will need to work to keep in the slipstream. A semi also can't stop on a dime, and you have much better stopping power than he does. So the risk of slamming into the back of the truck if he brakes is low, but you need to pay attention to the spacial relationship as you ride.
Of course, this all assumes a level road and close drafting. Once you hit even the slightest grade, you will fall back a little, catch the back edge of the slipstream, and the ride is all over.