To determine once and for all what the peg heights are, go to this link:
http://www.usfirst.org/roboticsprogr....aspx?id=18763
Download the Game Specific drawings for all the playing elements on this year's field. Specifically, look at drawings GE-11000 through GE-11005. From these you can determine measurements from the 11 gauge steel base plate that sits on the floor up to the bottom of the pegs, or to the center of the end disks which are welded tangent to the bottom of each peg's pipe, or to the top of this disc. Since I don't know how you will plan on aiming at the pegs, I won't publish actual dimensions here, but you can easily calculate them.
Plans are also on this site for team-built pipes made from PVC plastic pipe. Be careful when you make the pipes out of plastic. Not all tee-fittings are the same size, and pipe doesn't always bottom out when glued into the tee-fittings, so be sure to measure the finished products when you make them.