You know, I don't think I would ever let my baby sleep on his stomach in my bed, even though I have a very firm mattress. Maybe if I took all the pillows off the bed, and I was awake.I do let him nap on his stomach in the co-sleeper when I am awake and can peek in on him. When Teddy sleeps in our bed - like he does most of the night -- he sleeps on his back or side, in the crook of my arm, or sometimes I'll lie on my back and he'll cuddle on top of me.
However, my children, each and every one of them, have HATED back sleeping and side sleeping, alone in a crib. Teddy is actually taking a record-breaking nap on his back right now; he's been asleep for about 20 minutes. Woohoo. If I let him nap on his tummy in his co-sleeper, he has been known to sleep for 2 or 3 hours. He'll nap in his carseat for an hour or 2. On his back in the co-sleeper - usually 20-30 minutes maximum, and often he'll wake up nearly instantly.
For me, once my baby starts to roll over by themselves, I figure it's a moot point, and will put them down on their stomach to sleep at night. We have always kept our babies in our room in a portacrib- so it has a super-firm mattress. Still, I struggle with the guilt of going against the APA recommendations with each baby.
Now that I think about it, when I had my older children, co-sleeping was pretty much frowned-upon, too. And when my parents had me -- back sleeping was a no-no. The theory was that babies would choke on their spit-up, so they were safer on their stomachs.