See [this Meta report](http://meta.stackoverflow.com/questions/101464/eeeek-where-did-the-chat-top-navigation-link-go) for details, but to summarize:

To help promote the visibility of blog posts, **during the first 24 hours after a new post is added to your site's blog, it will replace the chat link in the header**. Afterward, it will be back to business as usual.

In Server Fault's case, you have two blogs. The main Server Fault blog is linked on the main site, while our general Stack Exchange blog is linked on your Meta site.