Just come to post about this same thing... I knew there would already be a thread on it cos it's an ongoing issue!!
My DD1 (now 2.6) would not sleep on her back for longer than 10 mins at a time. Not during the day, not at night, we cracked very quickly and let her sleep on her tum.
DD2 was born on Saturday morning, and I was determined that this time I would push harder to get new baby to sleep on her back since I was so worried all the time about DD1. Well, after 2 nights of absolutely no sleep unless baby was brought into bed with us, we cracked at 4am this morning and put her on her tummy. Finally, some sleep for everyone.
If my choice is between co-sleeping and baby on tum, I'm afraid I'm going to have to go with tum, because my DH sleeps very heavily, and I worry far too much when she's in bed with us that he'll squish her, so that means I don't sleep!!
It's just so hard... you know that sleeping on baby's back is safer, and you really want to do it, but if baby really literally will not sleep that way, what can you do?
Has anyone tried to persevere with a reluctant back-sleeper and managed to get them used to it?