I slept on my back until I was about 36w last time, but I had to have pillows under my knees. Another person on this site slept on their tummy until about 30w. Try not to worry too much.
As Kate I think it was that posted the positional thing is only important in some pregnancies in the later stages. And I still maintain - sleep in the way you can! If its on your back with pillows all round you - all power to ya!
If I sleep in an uncomfy position for bubs, it soon lets me know by sticking a foot in an uncomfortable space.