When +1 launched in March on Google’s organic and paid listings, Google said it would look at +1 click data “as a potential signal to improve search quality.”
A new +1 button FAQ explains this in more detail :
Content recommended by friends and acquaintances is often more relevant than content from strangers. For example, a movie review from an expert is useful, but a movie review from a friend who shares your tastes can be even better. Because of this, +1′s from friends and contacts can be a useful signal to Google when determining the relevance of your page to a user’s query. This is just one of many signals Google may use to determine a page’s relevance and ranking, and we’re constantly tweaking and improving our algorithm to improve overall search quality. For +1′s, as with any new ranking signal, we’ll be starting carefully and learning how those signals affect search quality.
That FAQ page also explains another SEO impact :The +1 button might get a page crawled or re-crawled :
Once you add the button, Google may crawl or re-crawl the page, and store the page title and other content, in response to a +1 button impression or click.