Just remember, it's mainly Google that doesn't follow nofollows, I believe yahoo still counts those urls (in the answers.yahoo.com) as follow in their own SE.
It was nofllow from the beginning I think and in fact can't see the reason for linkbuilding on Y! Answers - You'll start spamming, you will get kicked in no time. Answers are good only as a source of content for autoblogs and smuggling CPA offers in links in the replies.
I use yahoo answers for one purpose: list building. Its a poor source for links not only due to nofollow but because its does'nt allow anchor text for keywords, which is essential for good seo.
Doesn't mean it's not useful (but not as useful as it could be), I've used it for link building.. you do get traffic from it and targetted traffic at that.
It really annoys me when people call links "juice" it's not a goddamn smoothie! To answer your question though, yahoo is nofollow, but it's still a good way to promote your own site. By answering relevant questions and leaving your link, there is a high chance that not only that person will visit your site but a bunch of people who were searching for the same thing through a search engine.
I did an experiment a while back to see what yahoo did with these links. I didn't submit my site to any SE's and I didn't have any toolbars turned on. In the end Yahoo was the first search engine that I saw hitting the robots.txt file and my site eventually started ranking good for some of the terms. Not to say that this method will still work, but it has worked for me in the past. I don't run spam though and that might always have something to do with it. The key with yahoo answers is to search for questions that posting your link would be relevant and helpful. For spam it simply doesn't work. When posting your link in the answer put it as a source along with one or two other links to a big site like wikipedia or about or something (it makes it seem as if you aren't just peddling your own links, even though you are). It's really much easier if you aren't pushing spam. Good luck, Bryan
Many people who promote yahoo answers as a source of traffic misunderstand its benefits and believe that links there are good for link juice. With no follow links, there is no link juice and the real benefit is when you have a most popular answer, people find it in their searches, clink on your link and you have incoming traffic.
I don't know if it has always been nofollow, because some older seo sites said it's good to use for backlinks.
I'm sure that Yahoo answers has always been NoFollow. If somone asks a question and you write a blogpost or something about it in your blog and give a link in your answer to your blog you can get hundreds of uniques like that