If you've ever listened to an "audio book," such as those files that can be purchased from Audible.com, you know that they provide a "bookmark" feature that works both in iTunes and on the iPod. If you stop listening to a one of these files -- for example, if you switch to another track, or if you sync your iPod with your Mac -- the next time you play that file it will begin playing from the spot where you left off.
Apple recently published an interesting Knowledge Base article noting that you can use bookmarking on any AAC-encoded audio file by simply using a file utility (such as XRay or FileXaminer) to change the file's Type code to "M4B " (including the trailing space).
We tested this procedure and it works as advertised: changing an AAC audio file's Type code to "M4B " allowed us to come back to the file later and pick up exactly where we left off. It even worked with AAC files purchased from the iTunes Music Store. In fact, using one of the utilities above, we were able to change the Type code on a large number of files at once, instantly turning them into "bookmarkable" tracks. Although this feature may not be a big deal for shorter songs, we welcome it for longer classical and jazz pieces, especially for listening on the iPod; although the "scrub" feature of the iPod's scrollwheel is helpful, being able to instantly pick up where you left off is much better.
Note that this feature doesn't work on standard MP3 files; you'll need to convert files to AAC, or re-rip them from the original CD in AAC format, to be able to use this feature with them.