Yahoo Answers gives nofollow backlinks but you have to understand that Yahoo Answers have many subscribers,people using the feed and this gives you a multiple backlins(Do follow) I stated this in my blog.
I use yahoo answers for my joke site it gives me great trafic. It went from 10 unique visitors and the higest i got so far was from yahoo is 102 unique and that day over 2,000 urls was looked at.