I already find it really uncomfortable to lie on my back, not really because of the belly but because of the added breast pressure when they flop to the sides. :) At some point you'll get to the time when being on your back isn't comfortable.
I'm also a stomach sleeper and I've found kind of half-lying on a pillow is the most comfortable position for me. Have you tried that yet? I ordered a My Brest Friend body pillow which should be here Monday that I'm hoping will help me get more comfortable.