We used the sling a lot with DD#3, but I can't imagine not being aware of her breathing - even in the sling, I could feel her wriggling, and almost always had a hand on her to feel her breathing.

I think it comes down to common sense. And to be honest, I would be very surprised if the 13 deaths claimed by the US Consumer Product Safety Commission were really attributed to the slings. 13 baby deaths from one category of products would garner a lot more publicity than this...