Alistair Lattimore posted an interesting question in the Google Webmaster Help forums about the Google Plus Button.
He asked how does the Google plus button comply with the crawler directives. If a crawler is blocked and if Google uses +1 data for ranking, does the crawler still use that data?
Googler, Jenny Murphy, provides a quick answer:
The +1 Button interacts with robots.txt and other crawler directives in an interesting way. Since +1's can only be applied to public pages, we may visit your page at the time the +1 Button is clicked to verify that it is indeed public. This check ignores crawler directives. This does not, however, impact the behavior of Google web search crawlers and how they interact with your robots.txt file.
So there you have it.
Forum discussion at Google Webmaster Help.