Well as long as we're talking about CD's/DVD's
I don't think DVD's will replace CD's (as far as music goes) for a while. I read some DVD FAQ article or something on the internet that said the major record labels were uninterested at DVD at this point in time. And I think it would be a very slow conversion. It wouldn't be like one day you can go out and get CD's, and then the next day, all you can get is DVD's. I mean, they still sell audio tapes of most albums on amazon.com. Those have been around for ages! Plus, could you imagine the confusion of people trying to play the DVD's in their CD player? THe same size, shape, and everything! That's gonna be a mess!
And I love DVD movies, and the fact that nearly all of them come with a letterboxed format on one side. It's 500x better than VHS, which is a badly outdated format to begin with.