Thanks this post, written by Lauren Baker, we can have a summery concerning management of No Follow attribute by search engines Google, Yahoo and Ask (not Live unfortunately) :
As you can see, the GoogleBot will not follow the « no follow » links. However, Ask doesn’t know the no follow attribute and this search engine refuses to consider it. Yahoo has a particular behavior with No Follow attribute, because it indexes pages but don’t give trust to « no follow » pages.
Finally, No Follow does not mean that search engines do not see the pages which No Follow attributed links point to, it means in some cases (not Ask.com) link value nor referral attribution is given. However, algorithms often change with time and they can evolve… This is not a static information.