You guys know those glasses that get darker as you encounter more light (photochromic they're called), is that same technology being used in car tint yet? Man that would be sweet - dark tint on really bright days and at night it goes back to clear.
i suspect that tint film is too thin to use the same technology as you find in glasses, at least at this time. 3M however is likely working on just such technology. there is however a tint film that does change from dark to light, but not in stages. and it would need to be connected to your electrical system in order to work. it also take a fair amount of voltage to operate, though i think they have lowered the voltage requirements, but not enough to work in an automobile.
I've read a little about the LCD tint as well, where you pass current through it to make it clear. You can vary the voltage analoguously for varying levels of opacity - like a dimmer on a light switch.