Take note this only works for AAC-type files Even if the new iTunes can "bookmark" MP3s (e.g. podcasts) it seems to be restricted to the podcast section (so I think it's more of a software setting rather than file flag) I hope someone finds a way to make MP3s bookmarkable since I like having my ripped stand-up comedy mp3s resumable.
I'll post this again: 1.You definitely need to encode them to AAC. 2.Use this script (make bookmarkable) to convert them into m4b format. 3.They will then automatically show up in your Audiobooks folder when you transfer them to the iPod.
