We are tummy sleepers here.
With my first, I stuck with back and she was a horrible sleeper... HORRIBLE! When the second came along, I started the tummy sleeping when she could roll over. With the third, I tried back and within a week or 2, we switched to tummy sleeping. This kid... Well we tried back sleeping a few times, but even in the seats with an incline, she just won't have it!
There are several reasons that I am comfortable with the tummy sleeping. Long story short, we have no other risk factors for SIDS. (I have had adaquate prenatal care, I don't smoke, I drink rarely, I am not under 20. Baby was not premature, is never exposed to second hand smoke, and has no family history.) I do follow most other precautions (mostly because of common sense, not because they are actual warnings), like not overdressing baby, not sleeping with lots of blankets, etc.