There have been some good discussions on The Garage Journal board on the subject of using plastic for air lines as well as alternatives.
http://www.garagejournal.com/forum/s...ight=Air+Lines
http://www.garagejournal.com/forum/s...ight=Air+Lines
Also, some good general info on air lines and design here:
http://www.oldsmobility.com/air-compressor-piping.htm