The human ear in combination with the brain is a marvelous and mysterious instrument that can distinguish a certain sound conveyer of information from background noise. A sound nullifier systen has not yet been devised to make those distinctions and an efficient system would make us all deaf.

I believe the 2005 Acura RL has this feature. I would be surprized if it was the first...

IIRC, the R&D arm of Lotus (yes, of sportscar fame) started working on just this sort of thing over a decade ago, essentially picking up ambient sound from inside the cabin and playing it back phase-inverted via the stereo system. As suggested by "sand" above, the tricky bits involved placement of pickup mic's WRT the occupants' ears and the stereo speakers, and getting the system to distinguish the "noise" to be phase-cancelled from the "signal" (external traffic cues, in-cabin conversation or music) to be retained.

That could have a significant impact on driving safety.

Aside from sirens and horns, there are other things I look for while driving: high wind noises, unusual rhythmic sounds from the tires, knocking from the engine, audible vibrations...right down to the vague "unusual sounds."

Get a BMW... no fancy system, just good design (as I understand)

you can pick up DynaMat from most car audio retailers and its simple to install and not very expensive. it comes in many options and has a great Rep. i used it in a 94' F-250, $200 later and about 6 hours of work and i cant hear anything but music while going 80-mph down the free way. my advice dont waste your money on a new car. this stuff works