UPDATE: A post regarding this incident from apache.org is available at https://blogs.apache.org/infra/entry/apache_org_downtime_initial_report
As of this moment, Apache.org is reporting that SSH key associated with its US servers has been compromised and are shifting all traffic to their European mirror.
Details of the attack/compromise are few at the moment, as this is breaking news. It is worth remembering however that a compromised SSH key led to in-the-wild exploitation of Linux based systems exactly this time last year, for the purposes of installing rootkits. Keep your eye on how this story develops.
Apache servers account for around 50% of all web servers in the July 2009 web server survey.