I am hauling my bike a couple of thousand miles Without the seat and post in.  Any suggestions what to plug the tube with?
When I've had bicycles shipped, its generally just been left empty (on a used bike), or had a little plastic plug in it (on a new bike). If you do want to plug it with something (say, you're storing it without the seatpost for some reason and don't want any critters in there), you could plug it with some news paper or something which you can easily remove or cover the hole with a bit of duct tape or something -- It's just a pipe. 

I'm going to guess the question is really how to package a bicycle for moving and the easy answer is to use a cardboard bike box. Some directions are here, and most bike shops will package a bike for a small fee and even ship it for you. Normally, you remove the seatpost, the wheels, take off the handlebars, pedals, and then just arrange them in the box (without disconnecting the cables). Then, put all the things back together when you get home (note that the left hand pedal is opposite threaded).