After update (to 4.2) iOS says I have 723 songs, but iPod.app says "No Content"
I am speculating, that this is what happened: I did the update on another computer (not the one my music is synced to), which invalidated the copy protection of my music library, so when I plugged my phone back into the machine where my music library is stored it did the deed for me. That is what seems to have happened anyway.