I almost used a much more provocative title: iBooks supports EPUB 3 but then we would have to have a complicated discussion about “What does ‘support’ mean?” and even, “What does ‘EPUB 3’ mean?”
Keep in mind, for example, that an EPUB 3 file is not required to include anything besides text and simple formatting, and might be nothing more than EPUB 2 content in an EPUB 3 shell. Such a file would be much less difficult to “support”.
And also remember that supporting some aspects of a file format does not necessarily imply support of the entire file format. The EPUB 3 format itself is pretty loose about requiring what conformant systems must support.
But, now with all those caveats out of the way, I'm happy to say that it's true that you can open the EPUB 3 sample files available from the IDPF with iBooks.
And those files aren't particularly complicated, or replete with EPUB 3 features, but I did notice some interesting things.
First, there are three EPUB 3 files to look at so far, Moby-Dick, Wasteland, and Creative Commons - A Shared Culture. Only Wasteland has a toc.ncx file. The Shared book has no table of contents at all. But Moby-Dick (created by Dave Cramer of Hachette) uses the new-fangled XHTML style TOC:
and iBooks displays it beautifully:
This is a big deal if you ask me, as I'm not relishing having to create two separate annoying TOC files, one for new systems and one for legacy ereaders.
The Shared book includes video and audio, and scripts to control them, but it seems like the only scripted enhancement that partially works is the play button (and it works much better in a non-scripted way).
The Moby-Dick book includes media overlays, ostensibly for narrating the text, but they don't seem to work yet.
We're not there yet, but the fact that iBooks supports EPUB 3 in some fashion is at least a beginning. Now we can begin testing what is possible, what is supported, and what we can create.
Note that Readium, the IDPF encouraged extension for Google Chrome, also supports EPUB 3. It's definitely worth downloading and playing with.