I just gave it a try at: http://recaptcha.net/learnmore.html
In one case (among others) i was given the image that should read: relations ascrib
, and when I intentionally entered: relati0n5 a5crib
(by substituting o, s with 0, 5), it still passed.
That wasnt gonna give site owners anti-spam assurance was it? -- while the bot does not have exact idea how letters should read, neither does the verification!
Here's an idea: instead of giving out two captcha string challenges with indeterminate open sesames, it displays the first string with definite answer, followed by the second one with some uncertainty at the verification end -- whose purpose is to help to digitize books.