Perhaps you just have gotten better listings in the Google / Yahoo search engines in general. Typically my referrer report reveals my traffic from Google.com to outpace Yahoo.com slightly more then 2 to 1.

However, if I also add the traffic from the google.other_countries that is even higher.

in seven days it shows -
2385: http://www.google.com/
2235: http://images.google.com/
1109: http://search.yahoo.com/
267: http://www.google.de/
190: http://www.google.fr/
188: http://www.google.ca/
165: http://www.google.es/
164: http://www.google.it/
161: http://images.google.co.uk/
etc etc etc

I'll tell ya. Probably the best thing you get from a listing in the Yahoo directory isn't the traffic, it is the improved Google page rank.

Steve