Google announced they have updated their robots.txt testing tool in Webmaster Tools. The update brings you a few more features including:
(1) Highlights which line in your robots.txt file is blocking a specific page.
(2) Make test changes to the robots.txt tool and test it before you make the file live.
(3) Google will also show you older versions of your robots.txt file to see past issues.
Here is a screen shot showing off the highlighting feature:
Google Webmaster Tools Updates Robots.txt Testing Tool