I didn't realize Google Reader was being integrated into the SERPs either. I use Opera's client so I'd never see Google Reader even when I do logged in searches.
That gets me thinking. Signals from Google Reader may be a rarely discussed ranking factor, even for non-personalized results. There's got to be some good engagement data there for Google, don't you think?
Google has complete and free access to Google Reader, that much is uber obvious. The more subscribers a url has would seem to make your url more "popular" but it is especially relevant when the user has subscribed to it themselves, which is most likely why they are displaying it, to add to their personalized search features. A Google Reader subscription probably is a ranking factor with as much value as a social bookmark has.
GWT also shows Subscriber Stats under Your Site On The Web, but those are for RSS feeds available on site, not from Google Reader.