I've just heard that Apple released a new iBookstore Assets guide (4.7 Rev 4) this week. The guide is supposed to have instructions on how to enable the TTS functions of iBooks. There's certain code that you have to embed in your ebook when you make it. If you're an iTunes Connect member (more info here), you can download it from the Deliver Your Content section.
There's also a new version of iTunes Producer available. One of the improvements listed is "common EPUB errors can now be fixed automatically". Liz Castro, my source, isn't sure what that means. I'm waiting to hear what she says about it when she gets a chance to check it out.