I agree - now in my 26th week, I still often sleep on my back with no ill feelings at all, and did so with my first bub. i was also told you will know when it is time IF AT ALL. I continue to do light weights and exercises on the swiss ball on my back and feel absolultely fine. It is when I am lying on a hard surface (like on a yoga mat in class) that it feels BETTER on my side, but I have never felt dizzy or sick.

hope that helps - listen to YOUR body! everyone is different.