aristotle - 8:04 pm on Dec 26, 2011 (gmt 0)
Well I don't know for sure how Google could use +1s as a ranking signal. But if your site and a competitor's site both have the +1 button installed, what I think could happen is that the Google algorithm could compare the percentages of visitors who click it on each site. If one site's button gets a significantly higher click rate, then it might get a small rankings boost at the expense of the other site.
But if one site doesn't have the button, then it would be harder for Google to compare them. That's what I had in mind.