iTunes isn't available for Linux but free replacements have been written which can do most of what you'd need (though you might want to initialise the iPod on a Windows machine you can put songs onto it under Linux).

I suspect that it can be booted from an external hard drive although USB drives do suffer from a slow transfer rate.

The main problem with a Linux laptop as opposed to a Windows laptop is that in most cases the laptop was not sold with Linux and there is patchy support but that won't apply to you (it applies to those of us who buy more expensive laptops to run Linux on).

Linux will only run some Windows programs (but then again, there are a lot of Windows programs that won't work on Vista either) with DirectX games being the least likely to run and hardware that only has Windows drivers probably won't work (but with a laptop designed to run Linux that shouldn't be an issue with what is built-in) so if you have a Windows only printer you might have trouble. Some of them have been reverse-engineered so that you might be able to get them to work and a lot of manufacturers have supplied Linux drivers (or just information that allows volunteers to write the drivers) so hardware support under Linux is actually pretty good overall.

You carry it on…but you are supposed to pay a fair price for it to avoid piracy..

Whether you are copying from a "Live" CD or doing a conventional installation, the process allows you to specifiy the drive(s) and partition(s) on which you wish to install Linux.

The biggest complication (and really the only interaction between Vista and Linux) will probably be the bootstrap sequence. In a typical "dual boot" configuration, the Linux bootstrap is the primary bootstrap, and you choose your Windows OS entry from the Linux boot list if you want Windows.

In your case, the Linux bootstrap will be on the removable drive, so you will probably have to select which OS boots by using the BIOS boot selector (hard drive for Windows or USB/Firewire for Linux).

It may also be possible to have USB/Firewire be first priority (allowing you to use a standard Linux dual-boot configuration when the portable is connected) and the hard drive second priority (so that Windows will automatically start, possibly after a short delay, if the portable is disconnected).