There are different levels of ukemi and it can be practiced for different purposes.
Unlike Lynn, I am not always 100% selfish, although that type of training has it's place.
Unlike Szczepan, I don't believe that uke is never wrong, although that type of training also has it's place (http://www.aikiweb.com/columns/thegr.../2005_10.html)
Teacher uke's are great, and it is my responsibility to be that for many of my partners, particularly when they're lost or confused.
What's really missing is a discussion of how much time each a person should spend in the different types of ukemi (for personally developing technique, for teaching your partner, for challenging your partner, etc.) to develop one's own training and one's partner's training in the most efficient way possible.