Google Operating System wrote about a new personalization feature Google is testing, named Preferred Sites. How does it works? You go to your search preferences and enter in a list of domains or sub domains that you prefer. If Google finds a result from any of the sites listed in your preferred list, Google will bump up that listing to the top of the search results and label them as such.
Here is a screen capture from Google's preferred site help document.
This is not available to most people, but it is a limited test for some users. If you see the preferred site option in your Google search preferences page, that means you are in the test. Clearly, this is an extension of SearchWiki, which is now the default.
We have two threads discussing the impact this might have on SEOs. One thread is at Sphinn and the other is at WebmasterWorld. And this will impact SEOs just like any other personalization feature and similar to how SearchWiki will impact SEOs. In short, most searchers will be clueless to this preferred site preference, even if Google flashes it in front of their faces, like they do with Search Wiki. So we will see how much of an impact this makes being that (1) it is a test right now and (2) being that users might not know how to use it.