It's my understanding that all 71XX and 87XX models have polychromatic LED's.
None of the 6XXX, 72XX, 75XX, 77XX series have them.
You could, in theory, make an application that blends the LED in a continuous rainbow from violet->blue->green->yellow->orange->red in response to a specific meter, such as battery level, or signal level, etc. Kind of like an "alert" status.
BlinkBerry does this in a crude kind of way, but it could be done in a more interesting and useful way.
Another overlooked part of the BlackBerry API is that you can turn the backlight on and off -- something I'd like to see used in more applications for alerting (i.e. reminders, etc)