Plasmas will never reach a point that everyone is buying them. The technology is not ideal for several reasons, all of which are being discussed here:
The picture isn't very good and will remain poor due to the technology involved. I do believe thin panel televisions will eventually be very common but under a different technology, perhaps LCD or OLED, but Plasma is merely a stop gap.
That said, if you want a speaker to appropriate match a plasma television, Magnapan makes a series of very good panel speakers that aren't much different in depth than a Plasma. Additionally, there are some fabulous in-wall speaker companies which I assume would become more popular, to a degree if thin panel televisions saw explosive growth.
However, I don't think I'd want a blank wall with a flat panel and some invisible speakers for aesthetic reasons. It would have to have some furniture in place simply to make the wall less boring, and you will always be able to place a fine center speaker in such an arrangement.
Of course, personally, I'd rather have a projector anyway.