The bed bounce is reported in only a few percent of tundras, but mine does it a little at certain speeds on certain freeways. Other than that it's nice.
I believe it has to do with the dampening frequency of the shocks and the stiffness of the tires.
Some guys say that when they replaced the stock tires with a different brand the bouncing went away completely.
Make sure you keep the tires at 30/33 psi all the time following the sticker on the door jamb. If your truck is one that bounces real bad, here's one person's fix:
http://www.tundraheadquarters.com/blog/2008/08/11/bed-bounce-treatment-the-willybar/