If you want to shuffle the whole thing you can go to "Playlists" and select the playlist that corresponds to the name you have given your iPod in iTunes. (My iPod is named Blackout, so the playlist in the interface for all my songs is called Blackout) Next, press "SCN/RPT" untill "ALL RANDOM" is displayed. Then you should be shuffling all songs!
One word of caution: the head unit in the SL gets shuffle amnesia every time you turn off the car. :roll:
It will remember what song was playing but not remember its place in the shuffle. so every time you get in the car you will have to remember to pres "SCN/RPT" twice to get to "ALL RANDOM" unless you want to now hear all your songs in artist sorted order and not shuffled. This will make a new shuffle so you may hear repeats.