Just read a post on aiaConnect, Going Native, that ties in really well with my last post. They discuss some new research that indicates that native bilinguals actually hear the same sound differently in different contexts. In this case sounds in different places along the ba-pa continuum were identified as a native Spanish speaker would in a Spanish context, and as a native English speaker would in an English context.
It reminds me of when I was learning French and had to listen to hours and hours of my own voice recorded, and then the native speaker, and then myself, and finally had these breakthrough moments where I could hear the difference between my attempts and the perfect Parisian accent. In the beginning even Je and J’ai sounded the same to me. Really, it is just another challenge I am happy to spare my baby if I can–and it reminds me that I had better make sure to give him some input with proper pronunciation!