Just like how you can't see the "north pole" of a large sphere when standing close to it, the sphere is being clipped for the same reason.
You must calculate the distance for a plane that intersects the front of the sphere, not its center.
In essence, the value you calculated represents the minimum distance to the front of the sphere. To find the minimum distance to its center, simply add the sphere's radius to this quantity.
(Please note that this method provides a conservative approximation due to the sphere's curvature.)
UPDATE: Here is the precise solution.
var dist = radius / ( Math.sin( camera.fov * ( Math.PI / 180 ) / 2 ) );
fiddle: http://jsfiddle.net/x98Fk/3/
The vertical field-of-view of the three.js camera is used in this calculation. Therefore, the result pertains to the vertical direction. In cases where the window is wider than it is tall, additional adjustments may be necessary.