My setup similar to SL's although i refuse to use iPhoto as i really can't use to it, I do use Lightroom and if i'm editing an 'event' i will store the files on my laptop, but as soon as i'm done they are moved to an external drive.
iTunes lifts all my music from another PC via a share, i do allow the podcasts to be stored locally (it is a laptop so who knows where i'll be when i watch it) until i've watched them then they are manually slung to another drive.
TimeMachine has to be the easiest way to back up a mac, although i'm sure you could knock up an Automator in no time at all to move files if you wanted full control.
If you are ever curious where all your space has gone i use
http://grandperspectiv.sourceforge.net/ on my mac, quick and easy graphical representation of disk usage.