I have an issue with iTunes (which I have to say is the worst media management tool I've ever come across). I have a bunch of songs that are listed under Device / Music (I'm referring to folders down the left hand side of iTunes), but have since been deleted from my iPhone 4 (and incidentally, my computer). Now under iTunes I can't delete a song from this directory, but I can under Library / Music, which of course doesn't get into my iPhone.
Problem I have though is after reinstalling my laptop iTunes has syned everything in Device / Music and 'dumped' it on my iPhone, yet my iPhone is empty! Now it says I'm using 8.3GB and my capacity is blown.