As mentioned above audiobook bookmarks can be made manually with the audible app as a stop gap measure.
Definately an ios5 problem and not an itunes problem although it would be nice if they fixed iTunes to show album and audiobook art again.
I have a test audiobook that works ok on second gen ipod touch and an older 120 gb ipod. Take that same audio book and sync using the same itunes to an iPhone 4s and to an iPad 2 with ios5 and audiobook bookmarks no longer work.
I've taken books we've purchaed from cd and read them in combined tracks with audiobookbinder. Book parts are no more than 4 cds each. This program combines the individual tracks and makes a single m4b file which shows in itunes as an audiobook with the remember position checked.
Audiobook bookmarks are lost when returning to a book with an automatic saved position bookmark. Leaving a book for a song and then returning to the book, sometimes you can view the saved position for a split second. Then the book part starts at the beginning.
The squeeky wheel gets fixed first. There are a lot of ios5 issues that Apple's beta testers missed. I recommend that if you want this functionality back to submit bug reports to Apple. http://bugreport.apple.com