From October 2005 to January 2006, I experienced a huge raise in traffic from Yahoo (more then 300000 lakh visits per month for a semi-large site) although google traffic dropped to 500 unique visitors only.
Yahoo traffic was coming to www.domain.com/forums/ which was wrong for a sub-domain

I corrected this issue somewhere in end of February 2006, 301 redirecting the /forums/ to appropriate subdomain, last month I found google traffic was backup with more then 3000000 unique visits :-)
uptil last month yahoo traffic was gone down completely with only less then 500 visitors.

Now since last few days, I have found yahoo slurp spider actively indexing the subdomains and a slight increase in yahoo traffic as well.

I would request some suggestions to gain back my yahoo traffic and keep both google+yahoo sending me unique visitors.

Thanks.