From http://groups.yahoo.com/local/service.html

Dear Yahoo! Groups Members,

The Yahoo! Groups service will be down Friday March 15 9:00 PM PST
(GMT-8) as we move our servers to a new facility. We expect the
service to be back up the morning of Sunday March 17.

During this time the web site will be unavailable and email will not
be delivered. (Some users may experience email non-delivery notices,
but all email should be delivered once service has resumed.) Please
note: once the service is back up, there will be email delays due to
backlog. We expect these delays to last no longer than 1 day. Please
do not re-send email to your group as it will only add to delays.

We apologize for any inconvenience this may cause.

The Yahoo! Groups Team