Are all eBooks in the iBookstore DRM protected?

Yes.

(Not all books are DRM protected.)

Per the official iBooks FAQ...

How can I determine which books are copy-protected (DRM) versus those that are DRM-free?

iTunes can show you the kind of book you have by selecting the book and then choosing File > Get Info.

• If the Kind is marked as Book, this is a book you downloaded from the Internet.
• If it is listed as Purchased Book, it is a DRM-free book downloaded from the iBookstore.
• If it is listed as Protected Book, it is a copy-protected book from the iBookstore.

Only "protected books" have DRM protection.

Whether a book in the iBookstore has DRM is up to the publisher. Two publishers I know of never do DRM, O'Reilly and Take Control, and there are others too.