I was told a couple weeks ago that its not a good idea to sleep on your back after about 15 weeks, because of the same reason Alan stated.
I am not comfortable on my back anymore anyway, and only really find it comfortable when on my side. Also, my baby seems to go crazy and kicks ALOT when I lie on my back. I dont know if its just that I can feel his/her movements more on my back, or that baby doesnt like it. If I do lie on my back, I need to put a small pillow under my back for support otherwise its way too uncomfortable.