One of the very interesting new features that Apple has added to iBooks 1.3 is support for Media Overlays as specified in the EPUB3 spec. According to Lawrence Furnival, it works with Fixed Layout EPUBs using SMIL files that link the audio to the text, using a timeline and CSS to highlight particular words as they are spoken. It's pretty exciting.
I haven't figure out how to do it yet, but as soon as I do, you'll be the first to know.
Meanwhile, here's a sample. (I've been hearing that some people are having trouble getting it to work. I didn't.)
Sample Read Aloud book “Splat the Cat” from Liz Castro on Vimeo.
It looks like there are 42 titles available immediately. I found a link to “Read and Listen” on the Featured page of the iBookstore: