incrediBILL - 10:31 pm on Aug 2, 2012 (gmt 0)
Not that anyone seemed to be interested in the best tip for testing your bot blocker on demand, but Twitter also claims that Twitterbot honors robots.txt!
Twitter's crawler will respect robots.txt when scanning URLs. If a page with card markup is blocked, no card will be shown. If an image URL is blocked, no thumbnail or photo will be shown.
Twitter uses the User-Agent of Twitterbot/1.0, which can be used to create an exception in your robots.txt file. For example, here is a robots.txt which disallows crawling for all robots except Twitter's fetcher:
Side note: I also use Twitter Bootstrap [twitter.github.com] which I highly recommend for building responsive design sites in validated HTML 5. It was easy to learn and I deployed it same day.