http://gamedusa.blogspot.com/2007/05/drawing-circle-primitives-using.html

circle.frag

varying vec2 screenPos;
varying float radius;

uniform sampler2D tex;

void main()
{
    float dist_squared = distance(gl_FragCoord.xy, screenPos);
    // Sphere shaded
    if (dist_squared > radius)
        discard;
    gl_FragData[0] = gl_Color;

//  gl_FragData[0] = mix(gl_Color, vec4(gl_Color.rgb, 0),
//      smoothstep(15., 16.0, dist_squared));
}

circle.vert

uniform vec2 windowSize;

varying vec2 screenPos;
varying float radius;

void main()
{
    gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
    gl_PointSize = 32.0;

    // Convert position to window coordinates
    vec2 halfsize = vec2(windowSize.x, windowSize.y) * 0.5;
    screenPos = halfsize + ((gl_Position.xy / gl_Position.w) * halfsize);

    // Convert radius to window coordinates
    radius = gl_PointSize * 0.5;
}