Since this topic has come up semi-frequently here, I thought I'd point an excellent gamasutra post on the topic of whether or not it's ok to use celebrity faces/images/voices, etc in games. And it's by an actual attorney.
This is a pretty "wonkish" article (eg it uses phrases like "to prevent these mischiefs, the law will allow.." :)), but it does an excellent job explaining what the main issues are.
