The AAC *.m4a files after transfer to the iPod have completely wrong track length. For example, 3:38:21 instead of 2:25.
The playback still works fine, you wouldn't notice anything is wrong until you look at the screen.
*.mp3 files have no such problem.
The issue started in 2.1.2 for me, and wasn't there in 2.1.0.
Using Arch Linux and iPod Classic 160Gb.