I was going to suggest  changing your password.  It might be you were not hacked/cracked at all: Yahoo was.  I too use the site, and would be bummed if someone started sending from my address.

The mail forwarding with dummy accounts is a good idea.  I have a couple of public sets that forward to my more private accounts.  Still, I use the Yahoo account for business and public sites like Renderosity.

Hope you get things sorted out.
Rªnce