yay trig!

gistfile1.js

var G = google.maps.geometry.spherical;

var A = placeMarker.getPosition();
var B = event.latLng;

var radians = function(degrees) { return degrees * (Math.PI/180); };

// Compute the angle from A to B
var beta = G.computeHeading(A, B);

// The angle of the second triangle is 45 degrees plus the previous angle
var gamma = 45 + beta;

// The radius of the circle is cos(gamma) * distance from A to B
var x = G.computeDistanceBetween(A, B);
radius = Math.cos(radians(gamma)) * x;

placeMarkerHandle.setPosition(G.computeOffset(A, radius, -45));

for(var i=0; i<2; i++) {
	circles[i].circle.setRadius(radius - (i*3));
}