Bad defaults is one of my favorite user incentive tricks. Can't get users to upload their photo? Give them a super ugly picture as the default. They'll change it immediately out of disgust
Another tip - always set default profile gender to "Female".
Women are less prone to feeling alienated by gender, but guys typically get uncomfortable with being labeled as a woman and will change it
In a way I think this is less intrusive (and clever). These are merely "suggested defaults" that never actually get displayed on the user's profile. Actually using bad defaults may have the undesired effect of people not changing the bad default.