I think the only way you can do this is to browse to a play list select a song and push down on the IDrive knob. You will get a context menu with one of the options being "Random" -- select this. Now songs will be played from this play list at random.

The bummer is, this setting doesn't stick. the next time you fire up the play list, you need to set the "random" setting again.

I don't think you can do anything with the IPOD itself to cause playback to automatically shuffle.