While we have received all of the details on this matter, Prolexic suffered yesterday night (JST) due to some technical difficulties on its network, making it impossible for some people to access Mt.Gox website.
We hope to provide you with more detailed information as soon as they are made available to us.
Update on what Happened with Prolexic :
"The following report addresses the service degradation event during the time envelope exclusive from 14:20 UTC to 16:06 UTC on September 5th, 2012.
During the event envelope one of our upstream providers (TATA) experienced a faulty linecard failure in their Asburn router that peers directly with our Border router in the DCA Data Center. This failure caused all traffic that was being advertised through the Tata link to be dropped. The outage affected services both our Proxy and GRE solution.
Upon confirming the issue with TATA, the Prolexic SOC immediately escalated this to Network Engineering, and at 16:06 UTC the announcement of all routes through the TATA link was withdrawn. The removal of the TATA announcement resulted in all services returning to normal.
The hardware failure that was addressed by the TATA support team required them to issue remote hands to reset the card. After confirmation that the links were stable, Proexic Network Engineering restored all routes and announcements back to its default configuration. Traffic to Hong Kong, San Jose, and London Data Centers were unaffected during the DCA outage."