Two people in my address book with yahoo accounts appear to have had their accounts hacked and bogus mail sent from them, in both cases the email just containing a link. (The first was to a sites.google.com link, the second some shadier-looking URL). Both were sent to a list of about 20 addresses.

If you're a yahoo account holder, change your password or something.

If you receive an email from a yahoo account holder, and there's nothing but a link, DON'T CLICK ON IT.