I wanted to squash a theory before it even gets out there, well, I may be a bit too late for that... As we know, Yahoo! Adds NOYDIR Support To Exclude Yahoo! Directory Titles in Yahoo! Search Results. And at the same time, there is a Global Yahoo! Search Index Update. But the two are not related.
The NOYDIR tag will have zero affect on your rankings.
Tim Mayer commented at Search Engine Watch Forums saying:
The ranking changes are due to the index update and have nothing to do with the NoYDIR tag launch.
So if someone asks you, will it change my rankings. The answer is no.