Two days after the initial scare, the Yamanner worm has apparently been quarantined by Yahoo, and no longer poses a threat to the uninfected masses. While there are currently many people infected by the worm(a very small fraction according to Yahoo), the particular method of propagation the worm used has been eliminated, and Yahoo is advising that all users block e-mail coming from a particular address.

"We have taken steps to resolve the issue and protect our users from further attacks of this worm," Yahoo spokeswoman Kelley Podboy said. "The solution has been automatically distributed to all Yahoo Mail customers, and requires no additional action on the part of the user."
This could have been a much more severe problem, especially with the popularity of Yahoo's webmail, so it's good to see that someone was on the ball and able to get the situation resolved.