I had this same problem (on my iPod (video, 60GB), if i recall correctly it also happened with my previous iPod (2 gen, 20Gb))
And I have not yet used AB (Ill probably buy it soon
It happens to me a lot, often when the bookmark is located at a position over 3 hours into the book (I have many long audiobooks)
It occurs like this (well, most often at least);
I listen to a book.
I pause it.
It goes to sleep.
I wake the iPod and simply press play.
It will then often play for about 30 seconds-2 minutes
and then loose the bookmark and skip to the main menu
(Apple has updated the Audiobooks in such a way that when a book finished (or at least one part) it will stop playing and return the iPod to the main menu (unless the audiobook is in a playlist where there are also non-audiobook files, it will then start playing those))
It is hard to reproduce and to determine the exact conditions under which this occurs.
Sometimes I can also, after I have awakened my iPod 'simply' reposition the playhead one or two times it will also jump to the 'end' (loose the bookmark) and start playing the next song/jump to the menu. I sometimes try this when I wake my iPod, to check if it will be jumping back, so that if it does i can quickly restart the audiobooks and set the playback position to where I was (it will then play normally). but sometimes it doesn't jump and it will still jump after playing for 2 minutes (which is quite annoying when riding on a bike.)
...thats just my 2 cents (I had already shared this info, though not as specific, with the developers, but I posted it here to share it)