When an iPod is connected, you can select , "Only transfer ticked songs" or something to that effect.
It's actually quite handy, since I've got a library of 128kbps AAC stuff for my Nano, and for listening through my speakers at home, I've got 256kbps VBR AAC tracks, I just untick the 256 tracks, and then all I've got on my Nano is 128kbps tracks.
Fantastic!