I enabled Twitter Cards, but when I look at the source code for a blog, it doesn't show the "twitter:site" meta tag. I do have the twitter screen name listed in the Twitter settings when it asks for "Twitter's via user screen name". I just ran the validator on Twitter, and it didn't indicate a problem, but it does say that it is required in the setup guide... Is there another setting I missed?

(On a side note, when the validator previews a page, it doesn't show the blog image, even though the image twitter meta tag is showing properly. That isn't your issue, I don't think, however.)