Doug Kaye has been bitten by the green pixel problem. He blames the display, but I don't think it is that simple. Here's why.
I too have been troubled by this problem since I got a 30 inch display. Over that time, however, I've had 4 different Powerbooks or MacBook Pros and they've exhibited different behaviors. One would give me the green pixels every time, all the time. No amount of rebooting or reconnecting the display would fix it. With two, I never saw the problem--not once. With the MacBook Pro I'm using now, I see the problem occasionally, but disconnecting and reconnecting the video cord has always made the problem go away.
I also have two 30 inch displays--one at home and one and work and they behave consistently on the different laptops. So, I suspect that it's more complicated than a problem with the display. I think it's an interaction between the video card and display. I think, for example, that my new Macbook Pro (LED display) has a different video card that my previous two Macbook Pros. Even so, the fact that Apple has failed to acknowledge or fix the problem is disheartening.