These noise-cancelling headphones from Hitachi Maxell are unlike many previous similar types since they don't require separate power. Instead they connect to iPods via the dock connector, and can suck on the batteries through there. They do manage about 20dB of noise canceling with an "Active Noise Rejection" system, but the design is curiously crippling: they can only connect to iPods (4th gen or later) and iPod touches—not the iPhone. And to draw power this way means you'll get reduced iPod battery life, which may be as much as 50%. And the iPod's volume control doesn't work, so you have to use the slider on the headphones. Weird, but these HP-NC20.IPs are only about $80 in Japan, so you may still be tempted. [AVWatch]
I think the concept is quite interesting actually.
If it uses the Line-Out instead of the signal from the headphone-out than sound the sound might be improved.
Also they could use it to better adapt the altered signal.
But the specs are a bit weak, only 20dB with active noise-cancellation (like Bitstuff said) is not good.
Every IEM achieves around 35dB when music is playing. Passively.