Actually, I just tried it. The songs didn’t disappear.
You’re using Windows, aren’t you?
Yes, this happened with Windows, but through iTunes. Ideally, iTunes shouldn’t have removed the filed from the iPod without warning. I can’t see why Windows is at fault here. I also have a Mac, with my sister’s iPod hooked to it. I’ll try it sometime.
I’d like to clarify that you need to delete songs from the computer from outside of iTunes for what I described, to happen.
Besides, how does one go about retrieving files from the iPod back to a new computer without resorting to dubious third-party hacks? This feature is basic in those MP3 players I mentioned. iTunes had this feature early on, but they removed it in future upgrades. This sucks too.