How do i find out the name of a person who has a Yahoo ID?

You could look at their profile and see if they put their name in there, if they didn't - try googling their screen name (many people use the same name for different sites) and see if you can find it associated with another profile.

Not really. As you notices, from some time directory service doesn’t returns any results when quenned – one could speculate probably (temporarily ???) due to discovery of Yahoo IM worm which most likely used it to harvest IDs in order to propagate itself. Still, even if directory would work, you would be able to obtain only information which was given to Yahoo by person (not necessary real one). Since lots of people submit fictional personal data for security reasons – you would never know are they real or not.