Doesn't the wheel also serve as a touchpad? (Sorry, iPod-deprived soul here.)
If that's the case, clean it the way you would any other touchpad. I use a lightly dampened (not dripping wet) paper towel. If for some reason that's not enough, I put a dot of soap on my finger, work in into the paper towel and clean the touchpad with that.
Be careful what you use ... don't use Lysol, for example, or anything too abrasive.