I've already alerted Chris about this. It began about midnight (technically, the cert expired at 11:45 pm).
For the time being - depending on your system/browser, you can tell your browser to override the certificate warning. I've been able to do this (on the Mac) with both Safari and Firefox. Chrome...no. If you're a Chrome user (on the Mac, at least), my recommendation is that, until the cert is updated, just use Safari or Firefox instead. Not ideal (I'm a Chrome user...), but it works.
On the PC... I don't know, at this point. I don't have a test bed here.