Bi-Directional iPod sync

I'd love to see some better syncing options for portable media players (iPods especially). The main idea is that if you connect a new iPod you don't have to erase the library and replace it with the local one but that it actually checks the players library against the local one and allows you to synchronize it bi-directional: Songs which are on the iPod but not in the local library are transfered to the local library and vice-versa.

This could be done by either just checking metadata of both libraries or by additionally checking things like filesize/md5 hashes...