I was told not to sleep on your back after 12 or 13 weeks, blocking blood flow as lestypuss said. Bad for you and baby. But, i am generally a side sleeper anyway, so wasnt a huge prob for me. Lately thou, i have found myself waking up more on my back, and being more comfy this way.
I did find cuddling a pillow helped me to stay on my side, or, i just sorta propped it under my right side, so i was sorta 1/2 on my back.
I am far more comfy on my right side thou.