Check out the section on Circular Arcs and Circles, from Ching-Kuang Shene's excellent computational geometry course notes:
[G]iven three control points P0, P1 and P2 such that P0P1 = P1P2 holds, if we choose w, the weight for P1, to be sin(a), where a is the half angle at control point P1, the resulting rational Bézier curve is a circle.
The second diagram on that page is particularly useful. If P0 is at (0,0), P1 is at (1,0), and P2 is at (1,1), then the angle at P1 is 90 degrees; half that is 45, so assigning weights w0 = w2 = 1 and w1 = sin(45) = 1/sqrt(2) will produce a circle.