As nobody outside Google really knows what improves your ranking and what doesn't, seo is largely a matter of deduction. You don't need to be a rocket scientist to deduct that the arrival of the PLUS ONE button is likely to be an important factor in serp ranking. It dovetails perfectly with Google's strategies to return those pages first that are most likely to offer that information that you are looking for. What better way than to let readers “approve” what they found? This may be a major advantage to providers of genuine content compared to companies that can spend a fortune on advertising.