The certificate for this server is invalid. You might be connecting to a server that is pretending to be “swscan.apple.com” which could put your confidential information at risk.
As of 4:59 PM on 5/24/14, every Apple user is getting the same scary error.
Don't worry, you are probably not the victim of a man in the middle attack. It appears that the SSL cert for swscan.update.com, which is hosted at Akamai, has expired. This was probably due to a gap in the monitoring and management of the SSL certs provided to Akamai.
It is certainly possible that Akamai has been hacked, and a compromised SSL cert was installed. Not likely, though. To be safe, just hold off on any updates until Apple and Akamai get the cert updated. Alternatively, you can download updates directly instead of using the App Store. You can download all major updates here, and this server uses a different SSL endpoint that has a valid certificate: http://support.apple.com/downloads/
Pro tip for Apple's security team: Even though swscan.apple.com lives at Akamai, you should set up SSL cert checks in Nagios for all exposed HTTPS end-points that are in the apple.com domain. These are the sorts of things you want to get notified about 60 days in advance. In the unlikely event that Akamai has been hacked and the cert replaced, this type of monitoring would have immediately alerted you. Win-win.
Both Apple and Akamai should have been monitoring this SSL cert. I do not think anyone should lose their job over this. If anyone does lose their job over this, that would be a failure of management, not the person or team responsible for SSL certs. I think this should be seen as an opportunity for improving monitoring and business processes.
I tried sending an email to firstname.lastname@example.org to let them know they had a critical SSL cert that has expired. My email received an error in response: "Your message to email@example.com could not be delivered for the following reason: This group does not accept external messages."
Update: I received a response from Apple's Security Team. They obviously resolved the expired SSL certificate, and they've addressed the bounce issue I reported. Fairly promptly, for a holiday weekend.