O'Reilly Media begin selling some 800 titles in iBookstore, along with Microsoft Press titles. As always, there's no DRM on ebooks sold by O'Reilly, so it's easy to read the open-standard EPUB files purchased from iBookstore on nearly any device with EPUB support. Several titles that in print form included supplemental material on CD or DVD are being repackaged to more clearly indicate where that supplemental material can be found for ebook customers, and those titles will be added to iBookstore gradually in the coming weeks.
For now iBooks availability is limited to US and Canada. Unfortunately, for now iBooks doesn't support updates for ebooks, but each title includes information about how to upgrade your iBookstore purchase with oreilly.com for $4.99 to get access to additional DRM-free formats and free lifetime updates.
The iBookstore is of course included in the free iBooks app for iPad, iPhone, and iPod touch (iTunes link).