I promise I'm not totally trying to hijack your thread, but I have some pictures of my 26tbss, and I had a similar question. Mine does swivel, and the back panel (which typically for us is on the bed side) is a different material than that on the inside of the entertainment center TV compartment. One of the pictures shows the profile view, and another shows a spot on the bottom where you can indeed see that there is some plywood of some sort between the layers of "cardboard" coated with a wood veneer.
I'll post the pictures shortly, I'm uploading to imageshack. This is a shot of the bottom of the swiveling unit.