I'm all for improving textbooks with more interactivity and richer interfaces, but I think the iBook is just one small, problematic, step towards that.
Requiring a dedicated, expensive hardware platform seems ridiculous, and the licensing terms are beyond draconian. The last thing we need is more educational content locked-in to a proprietary single-vendor platform.
IMO it would be much more interesting if it had an open format, cross-platform support (including on low-end hardware like OLPC), seemless integration with the web and with other software (starting with compilers/interpreters/etc for CS).