I got the invalid(or unverified?) Certificate as well when I went to the site. But when I continued all was secure and fine. Maybe its something with the twitter feed as mad said. I dunno. But I feel good about it. There have been many times (not a lot!) that I've seen invalid certs on a trusted site. I just assumed it was that something wasn't "renewed" or in sync or something.
Its good that everybody brings up anything odd they see so that Isaac is aware of it. I'm sure he knows, but he can't see everything.
There is always bugs with new sites/major enhancements.