According to Wikipedias website we have found the following information regarding the current downtime:
On July 2nd, 2009, the entire web infrastructure for Authorize.net (main website, merchant gateway website, etc) went offline a few minutes before midnight Pacific Time. None of the over 200,000 merchants who use Authorize.net payment gateway were able to process credit cards. Authorize.net’s main corporate office and their customer service were closed due to the July 4th holidays in the USA. Reports from their technical support line indicate that there was a fire in their California data center  and that service should be restored by 10am Pacific Time.