No, literally. I guess their datacenter is on fire. 
So if you're having problems with them it's probably why. I'm not sure if they're officially back up yet, but Twitter is full of people having problems as this is posted.
Authorize.net is on fire!
Submitted on Sat, 07/04/2009 - 09:49
This was on Friday. Not sure how long it affected service, but it's back online now. Here's the two most recent news updates from Authorize:
(for some reason they don't list hours, just date)
July 3:
Downtime Update
The Authorize.Net platform experienced downtime due to an electrical fire which took place at our primary data center in Seattle, WA. This event also affected the backup datacenter. During this time, transaction processing, the Merchant Interface and the public Web site were all offline. The Authorize.Net operations team worked closely with the team from the datacenter to restore full functionality.
Please check back later for additional information, including instructions on how to resubmit transactions.
We apologize for this disruption in service.
July 3:
Downtime Follow-Up Information
All transaction processing has been restored via all processors. However we are still working to restore our CIM and VPOS services.
Once our systems went down, all transactions that were attempted received a failed transaction response. There is unfortunately no way to recover lost transactions that were submitted during the downtime, as the payment gateway did not receive the transaction information from your Web site, shopping cart or other application.
Automated Recurring Billing (ARB) transactions which did not run as a result of the downtime will be rerun tonight by the payment gateway. It is critical that you do NOT submit those transactions manually, as this will result in your customers being double billed.
In addition to the Merchant Interface, Authorize.Net is providing regular updates on Twitter. Please visit http://www.twitter.com/authorizenet to follow us.
Again, we apologize for the unanticipated interruption to the payment gateway service.
Wow, I'm glad I just took my site live the other night via PayPal Pro then