Have you tried adding a println(key) to the keyPressed() and keyReleased() callbacks?  If you can see a difference in which keycodes are being received that may explain why you're having problems which only happen on one platform.

You're approach does seem a bit overcomplicated (using a 526 bit array to track 4 keys).  I've pasted an alternative below that you might want to consider:

1. Joystick stick;
2. 
   
3. void setup() {
4.   size(50, 50);
5.   frameRate(50);
6.   rectMode(CENTER);
7.   stick = new Joystick();
8. }
9. 
   
10. void draw() {
11.   background(0);
12.   stick.draw();
13. }
14. 
    
15. class Joystick {
16.   int vertical, horizontal;
17.  
18.   Joystick() {
19.     vertical = 0;
20.     horizontal = 0;
21.   }
22.  
23.   void draw() {
24.     int x = (width / 2) + ((width / 2) * horizontal);
25.     int y = (height / 2) + ((height / 2) * vertical);
26.     rect(x, y, 10, 10);
27.   }
28.  
29.   void moveUp() {
30.     vertical = -1;
31.   }
32.  
33.   void moveDown() {
34.     vertical = 1;
35.   }
36.  
37.   void releaseVertical() {
38.     vertical = 0;
39.   }
40.  
41.   void moveLeft() {
42.     horizontal = -1;
43.   }
44.  
45.   void moveRight() {
46.     horizontal = 1;
47.   }
48.  
49.   void releaseHorizontal() {
50.     horizontal = 0;
51.   }
52. }
53. 
    
54. void keyPressed() {
55.   switch (keyCode) {
56.     case UP: stick.moveUp(); break;
57.     case DOWN: stick.moveDown(); break;
58.     case LEFT: stick.moveLeft(); break;
59.     case RIGHT: stick.moveRight(); break;
60.   }
61. }
62. 
    
63. void keyReleased() {
64.   if (keyCode == UP || keyCode == DOWN) {
65.     stick.releaseVertical();
66.   } else if (keyCode == LEFT || keyCode == RIGHT) {
67.     stick.releaseHorizontal();
68.   }
69. }

I use a 526 bits array to store the state of every key to avoid problems when multiple keys are pressed. It's a common problem in arcade games played on a computer because on the real hardware you can't input more than two directions at the same time but when you use a keyboard you can easily input three or four. In some arcade games on PC this "bug" can be exploited to make things you shouldn't. To avoid cheats based on that bug exploitation I've choose to do so by using the multiplekeys method available on the wiki.

Fair enough, I figured it might be something like that.  If I get a chance when I get home from work I'll have a look at your code on a Windows 7 system to see if I can recreate your problem.