I don't recall that precisely. But perhaps you are looking for the keywords quadtree or partitioning. Or perhaps not...
I also remember somebody posting recently about a new physics engine in the library section.

Dunno what thread you're talking about but in case you just need a sketch with lots of particles i thought i'd share this sketch that i made some months ago. 200k particles at 60 fps is fun :)
however adding physics slows it down like a mofo

1. import processing.opengl.*;
2. import javax.media.opengl.*;
3. import java.nio.FloatBuffer ;
4. import javax.media.opengl.glu.*;
5. import com.sun.opengl.util.*;
6. FloatBuffer fb;
7. class Particle {
8.   public PVector pos;
9.   PVector vel;
10.   float speed;
11.   Particle(PVector _pos,PVector _vel,float _speed) {
12.     pos = _pos;
13.     vel = _vel;
14.     speed = _speed;
15.     vel.normalize();
16.     vel.mult(speed);
17.   }
18.   void tick() {
19.     pos.add(vel);
20.     if(pos.x > width||pos.x<0) {
21.       vel.x = -vel.x;
22.     }
23.     if(pos.y > width||pos.y<0) {
24.       vel.y = -vel.y;
25.     }
26.   }
27. }
28. class ParticleSystem{
29.   ArrayList<Particle> particles;
30.   public int numOf;
31.   ParticleSystem(){
32.    particles = new ArrayList<Particle>();
33.    numOf = 0;
34.   }
35.   void add(Particle p){
36.     if(numOf*2 <1000000){
37.        particles.add(p);
38.        numOf++;
39.     }
40.   }
41.   void tick(){
42.    if(numOf ==0) return;
43.   
44.    for(Particle p:particles){
45.      p.tick();
46.     fb.put(p.pos.x);
47.     fb.put(p.pos.y);
48.    }
49.    fb.rewind();
50.    PGraphicsOpenGL pgl = (PGraphicsOpenGL) g; 
51.    GL gl = pgl.beginGL();
52.    gl.glVertexPointer(2,GL.GL_FLOAT,0,fb);
53.    gl.glDrawArrays(GL.GL_POINTS,0,numOf);
54.    pgl.endGL();
55.    println(numOf);
56.   }
57. }
58. ParticleSystem ps;
59. void setup(){
60.   size(800,800,OPENGL);
61.   background(0);
62.   noStroke(); 
63.   ps = new ParticleSystem();
64.   fb = BufferUtil.newFloatBuffer(1000000);
65.   PGraphicsOpenGL pgl = (PGraphicsOpenGL) g; 
66.   GL gl = pgl.beginGL();
67.   gl.glEnableClientState(GL.GL_VERTEX_ARRAY);
68.   gl.glColor4f(1.0,1.0,1.0,0.5);
69.   gl.glPointSize(5);
70.   pgl.endGL();
71. }
72. void draw(){
73.   frame.setTitle(str(frameRate));
74.   background(0);
75.   if(mousePressed){
76.     for(int i = 0; i< 1000;i++){
77.      ps.add(new Particle(new PVector(random(width),random(height)),new PVector(random(-10,10),random(-10,10)),5));
78.     }
79.   }
80.   ps.tick();
81. }

edit: come to think of it this might be what you're looking for (obviously that is where i found the way of rendering tons of particles very quickly)