Airlines were developed by Shimano in the late 90s as a bid to improve shifting reliability for downhillers. Basically high pressure air replaces the cable on the rear mech. Riders like Brain Lopes were used as test pilots. When finished it was an insanely expensive (limited edition) set to buy, costing around $1200. As you may have noticed, it didn't catch on...
As said they take a bit of setting up to get properly dialed in, but when they are they're a breeze to use. One tank of air (ca 120 psi) gives around 300 shifts I seem to remember reading somewhere, which is optimised for a DH run (or a cruise round town!
This brilliant site has the whole story if you're interested:
http://www.disraeligears.co.uk/Site/Shi ... 01%29.html