I don’t use audiobooks so I don’t understand what’s considered to be a full bookmarking implementation. This article:
http://blog.wired.com/music/2008/03/hands-on-with-t.html
says “Sandisk’s whole line of MP3 players now supports audiobook bookmarking and can treat non-Audible files as audiobooks.” I believe you before I believe Wired, though. 
