Are you using Safari in Mac OS X 10.5? I recently started getting certificate errors when I try to login to Comcast from a machine with that configuration. But it doesn't say the certificate is invalid or expired, it says it was signed by an unknown certificate authority. It started a couple of weeks ago, when I installed the Safari 4.0.5 update.
This was my work machine, and it was replaced yesterday with a machine running 10.6. I haven't tried going to Comcast from the new machine yet, so I don't know if the problem still persists there. I don't have the error on my machine at home running 10.6.