Pfui - 5:48 am on Feb 29, 2012 (gmt 0)
@ergophobe: I realize Pinterest's site-lifting isn't crawling per se. (Aside: Arguably, it's closer to selective scraping.) But the copying is done by bots.
So, along the lines of Archive.org denying access to sites' wayback archives if there's a robots.txt denial -- "Page cannot be crawled or displayed due to robots.txt." (emphasis mine) -- Pinterest (& others) could similarly respect siteowners' terms rather than require yet another tag.