Keeping music only on iDevice and not in the iTunes Library it syncs with Is it possible to sync music onto the iPhone and then delete it from the iTunes Library to save hard drive space, without having it be automatically removed in a later sync?
Yes. When you have the device plugged in, click on its icon in the navigation bar on the right, then in the main part of the window turn on manual management for the various types of content as shown across the top of the window.

It is, however, a very good idea to have some kind of a back up of your iDevice's content somewhere in case something bad happens with the device. External Firewire and USB hard drives can be had very extremely cheap.