Update: Visa's Web site is down.
The folks who launched an internet hacking campaign that apparently took down the website for Mastercard for several hours today, are now targeting Visa's website.
"WE ARE ATTACKING WWW.VISA.COM IN AN HOUR!" the twitter account for Operation Payback announced.
The co-ordinated distributed denial of service attacks have apparently been launched in retribution against financial companies that have cut-off WikiLeaks founder Julian Assange, who was arrested Tuesday in London.
Operation Payback has been linked in the media to the influential internet messageboard 4Chan, which has been blamed for attacks on other websites in the past.