I have a 7.1 preamp,is there a 7.1 dvd player?

Not yet. But you can split the rear channels and send them to both rear and sides.

Maybe a better question might be; is there a 7.1 dvd?

There are 7.1 DVDs but only through the digital outs.

Dang, I had no idea. Are these discrete channels? How many such discs are available?

They are actually 6.1 Unsound but they are the reason receivers converted to 6.1 with a rear center. Almost all receivers are now using 7.1 and creating stereo rear centers or back surrounds. Of course some have even more channels but they are simulated at this point.