From the Drupal site:

The time has come. We are going to turn off some services on drupal.org on Wednesday February 18th, 2009 from 6:00pm to 10:00pm UTC (what time is this in your city?) to perform the upgrade of the live site from our current Drupal 5 version to Drupal 6. The downtime is important to make sure that we have all data intact for the upgrade, and new data is not added to the site while a copy of the site is being upgraded. We scheduled a four hour downtime window, so that we can make sure that everything is working properly before we launch the newly upgraded site.

Read more here (while the site isn't down): http://drupal.org/node/374078