If you have more than one Macintosh or are just getting a bit stingey on disk space (seeing as you have 20-30GB of iPod tunes), you may be wondering how you can play back songs on your iPod while it is connected to your Macintosh.
Here are the helpful hints that I found to be not quite as obvious as I would have liked. Note: these have only been tested on the second generation iPod, so they may not all work on the first generation.
Playback in iTunes from the iPod's disk
This one is a bit annoying, but it works well. My scenario is that I have a desktop machine with a ton of disk and a laptop with less (OK, it's 60GB, but it's less than the few hundred that the desktop has), my iPod has 20-30GB of storage space, I want to sync the iPod with my desktop machine, but use iTunes to play back the tunes from the iPod connected to my PowerBook.
Now, you could just use one of the power-only connectors, or use the second technique (described below), however, I want it to play through iTunes, so that the Macintosh does the mixing between the music and the Sound Manager sounds.
If you connect the iPod normally, while it is synchronized with another Macintosh automatically, it will ask you if you want to nuke the data on the iPod and synchronize with your current machine. Obviously, you don't want to do this, so you click "No". However, that gets you the iPod music in a dimmed list. No way to play it. The key is to turn off synchronization by clicking on the iPod options button (leftmost icon at the bottom of the iPod window in iTunes). You will probably want to re-enable this on your main machine when you connect the iPod the next time, or it won't synchronize, but at least you will have the ability to play through iTunes in the interim. Unfortunately, disabling synchronization means that the last played and play count information will not be correct for songs played on your laptop.
Playback through iPod while charging
Of increasing concern due to the shorter battery life of the newer iPods is the ability to play songs from the iPod while it is charging. If you are comfortable with using the earphones, you can do this by turning off the "Enable Firewire disk use" option. Once it finishes with its automatic synchronization (assuming you have that on), the keypad works and you may play music through the headphones.
But, if you don't have that option turned on, or you aren't using automatic synchronization, you can also make the iPod playable by clicking on the iPod eject button (rightmost button in the iPod window in iTunes). Once the iPod is officially disconnected, it is in charging only mode until you reconnect it and you can use it for playback through the headphones (or line out).
These may all be very obvious hints, but being the geek that I am, I turned on automatic sync and Firewire disk mode when I bought my first iPod and haven't ever turned them off before my recent acquisition of a new 30GB iPod.