Unfortunately Apple has 'decreed' that none shall use an iPhone/iPod without the concurrent use of iTunes to manage the device.
Yeah this is what other people said but with the addition that you can't really. Not without a fair amount of hacking around in the iPhone. The iPhone's filesystem is not exposed to general usage and not designed to be used with Linux (EG no iTunes for Linux). While there might be a way (and a place) to copy your audio files (yes the iPhone plays generic MP3 and WAV files) to your iPhone manually, it is generally not supported.
What you may want to do is see if there is an iTunes equivalent application for Linux that will let you sync music/podcasts/etc. If you do please post that here, it might be helpful to others.