Are you going to put all 7 speakers in the walls, or only the surrounds and rear surrounds? What about your subwoofer, is it going in the walls, too? LOL. Maybe, you could do a combination: wall-mounted speakers (maybe dipole) for the surrounds, in-wall speakers for the rear-surrounds, and bookshelf/floorstander speakers and a strong centre for the fronts. You should still be able to get good sound without too much difficulty with the combination. Just my 2 cents.
cheers,
Ohmage.