Traffic circles can be quite daunting for cyclists, because there are so many vehicules (including cyclists) crossing paths, that it is a recipe for disaster. The general rule for traffic circles is that you must give priority to people already inside. There are a couple exceptions to this, namely the Arc de Triumph in Paris and a few others around Paris. Where they work in the opposite manner, people entering have priority. There are two cases to consider.
First : The traffic circle with one lane entrance into the circle. The safest course of action is to take the lane and enter as an automobile. Maintain the lane through the circle and exit at the desired destination. If making a right hand turn from the circle (or left for those brits and auzzies) taking the lane isn't necesiary because you are following the curb.
Second : In a multilane entrance (2 lanes, I would probably avoid any traffic circle that has 3 lane entry becaue of the sheer volume of traffic) taking the lane again is a possiblity. Take the lane depending on destination. Merge Left to go strait ahead or make a right hand left turn.
Some traffic engineers have wanted to put bikeways on the periphery of the traffic circles and then either controlling the entrances and exits with lights or having the cyclists yeild completely to cars. In either case, the result is not desireable. Primarily because it forces either more delays for everybody (or atleast the cyclists) and furthermore, it creates more opporunities for car-bike collisions because the cyclist must cross the roadway more times than if he simply acted as an automobile.