chromatic's blog post is a good read, but as he points out, it would actually be beneficial to have an extra layer between perl's internals and user modules to prevent XS code from accessing the internals. I don't disagree with the necessity for changing the internals periodically. It would just be nice to have a binary compatible interface that shields you from those changes...

The perl api mostly consists of inline macros, for speed. It should be relatively simple to design a semistable binary api: just provide out-of-line-compiled functions for each of the inline macros. There would be a slight speed penalty, which is why nobody has done it.