That´s what I hate about iTunes. 😡 It doesn´t remember which music you bought with your account. It wouldn´t be hard to offer that. Xbox Live Marketplace works that way for example. If you delete something, you can redownload it again later since the systems remembers that you have already paid for it. You wouldn´t have to worry about backing up your entire collection, if iTunes worked that way!
But if you have the music on your iPod and you log in with your account on iTunes, can´t it copy the music onto your computer? I don´t have an iPod so I´m not sure which way it works.
What about this scenario for example: Assuming you buy something on your laptop and copy it to your iPod, shouldn´t you then be able to copy that music to your PC/MAC too from that iPod? I´d actually like to know how exactly it all works. And what doesn´t work.