tedster - 5:36 pm on Sep 2, 2011 (gmt 0)
My assumption was a bit different. I'm thinking that when a +1 vote is registered for a page, then that URL gets added to Google's crawling queue if it wasn't already there - and it then gets crawled even if there's a Disallow rule.
For one, that process would be a lot less resource intensive that taking a full inventory of every URL on the web. And this process would only be a minor ignoring of the robots.txt protocol instead of a major violation.
Nevertheless, we'll need to be very cautious about any automated adding of +1 buttons across a website.