On Mon, 18 Feb 2008 12:08:17 -0800 (PST), wrote:
new plastic [pipe and fitting will repace the drain line easily.......
no biggie

This implies we can't use plastic for the discharge tube:
http://www.usinspect.com/WaterHeater...chargeTube.asp

Here's what it says ... (catch that last counter-intuitive line!)...
Donna

A discharge tube is a tube or pipe that is attached to the TPR valve that
directs the superheated water down to the floor and away from anyone in the
discharge area to prevent scalding or burning.

The pipe itself must be made of a material that is rated for both high
temperature and pressure, which includes most rigid wall copper or iron.
Also, the size of the pipe must match the opening size of the TPR valve
discharge (usually ¾ inch).

The tube must terminate no more than 6 inches from the floor or be directed
to the exterior of the home.

If the discharge tube is routed to the exterior, the pipe must discharge 6
to 24 inches from grade, with a downward slope to prevent the pipe from
clogging or forming a trap. Blocked discharge tubes will prevent the
superheated water from discharging and will burst.

Though counterintuitive, it is often recommended that the discharge tube
terminate next to the water heater so that any malfunction of the water
heater will be more readily noticed.