I'm 33 weeks and often sleep on my back, it'll become uncomfortable for you before it bothers the baby. Don't worry.
I usually wake up on my back but then I just reposition myself.
Just read this on baby center:
Don't be alarmed if you go to sleep on your side and wake up flat on your back. It happens all the time and shouldn't have any serious adverse effect on your baby's health. Just shift your position and go back to sleep.
heres the link:
http://www.babycenter.com/406_is-it-safe-to-sleep-on-my-back-during-pregnancy_1245287.bc
Me too so I'd like to know also!im 23 w+5d