Fiona my first DS was posterior also and I ended up with a vacuum extraction as well as an epsiotomy(sp). My second DS was also posterior but because he was my second I actually birthed him posterior as he didn't turn and all I used was gas, so it is possible to birth posterior bubs, with a bit of back pain tho.

Also if you have a homebirth, I'm sure the midwife would be able to get you in to a position where bubs wouldn't get stuck, but in that event if it happened then I'm sure that you would be rushed to hospital.

I hope that I've helped some.

hugs xoxo