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.