I've conducted an experiment with several friends in my 'Friends' Circle within Google+
To start I +1'd a clients site whilst signed in to Google. Then I Signed out and went on to tweet the URL for another clients website and finally facebook shared yet another clients website (within another industry)
I asked my friends to +1, tweet and FB share a URL which I gave them.
Now when signed into Google the +1'd website (normally at 9th in Google) is now ranked 3rd with a note under the SERP telling me my firend +1'd the site. The website which received the Tweets is now 9th on Google, it is normally 12th when not signed in. Also a note appeared under the serp saying my friend had Tweeted the URL.
The page that received the facebook shares didn't change in position!
All the people concerned all follow each other on twitter, belong to the same circle on Google+ and are friends on Facebook.
Not a very well set up or executed experiement, I made it up as I went along and put into practise within 5 minutes of conceiving the idea. But interesting results all the same.
If Google+ takes off and people adopt the trend of staying logged into Google when searching, Social Metrics are going to have a huge impact on traditional SEO.
Now all we need is someone to setup a 'proper' experiement.