Many webmasters complained that the title and description given to organic search listings when a site was listed in the Yahoo directory affected click through ratios and rankings. Now they can opt out of Yahoo using the directory information.
The format for the meta tag is:
<META NAME="ROBOTS" CONTENT="NOYDIR">
<META NAME="Slurp" CONTENT="NOYDIR">
Yahoo noted that to make these changes effective there would be an index update. Full information is available on the Yahoo blog.