I was poking around in webmaster tools this evening and found that the +1 section had some interesting data. It appears that during the default reporting period (1 month) Google has been experimenting with adding +1 buttons to the SERPs next to my site.
During this period, the reports show that they have shown up to 200 +1 button impressions for each of my pages in the SERPs. A couple pages have 200 impressions and the rest have less. Looks like maybe 1000 impressions of the +1 button total.
Another report shows that during this period I got 270 +1s "from other sites" and 0 "from your site" (I'm not putting the +1 on my site if I can help it). I would assume that most of those +1s are from the SERPs,
I'm hoping everybody will be willing to share the data from their sites and we can compare. I'm interested to know what a good baseline for +1 activity is and if this is a signal that could actually be useful to Google for ranking. They have certainly put lots of effort into mining out all the data they can from +1s.