Welcome to the club…
As Nick Mediati noted on Macworld this morning:
If you were tried to install OS X software updates this weekend, you might have noticed that you…can’t. As MacRumors points out, OS X users who attempt to install software updates through the App Store app are getting error messages that complain about an invalid certificate. The reason? Apple apparently forgot to renew the SSL certificate in question.
Nick continues with some explanation and an alternate if you *really* need to get those updates downloaded and installed: Apple neglects to renew SSL certificate, breaks Software Update in the process.