Today, one of the biggest payment gateways, Authorize.Net, went down because of a big power outage caused by a fire. Everyone is twittering like crazy, ranting and asking customers to order by phone.
Authorize.Net is also our payment gateway at work, but this wasn't a major issue for us. We just had to swap the payment processing implementation from Authorize.Net with the one that uses a Verisign Payflow Pro account, and that's it. We even didn't have to shut down the site because we have this set up via dynamic.web.config.
Really, you gotta love dependency injection in times like these!