A new update has been made to the Google webmaster guidelines document, which describes you should not block the Google ads destination URL with your robots.txt file.
The new text is under the technical guidelines and reads:
Make efforts to ensure that a robots.txt file does not block a destination URL for a Google Ad product. Adding such a block can disable or disadvantage the Ad.
The guideline before somewhat implies the complete opposite:
Make reasonable efforts to ensure that advertisements do not affect search engine rankings. For example, Google's AdSense ads and DoubleClick links are blocked from being crawled by a robots.txt file.
So what do you make of this new change to the Google webmaster guidelines?
This was spotted first by Menashe Avramov.
Forum discussion at Google+.
Update: The following day, Google has removed the language from the guidelines. I guess they decided it was confusing also?