reCAPTCHA has been around for ages. This isn't something new. Everyone knows about it. It's also widely implemented.
I would like to add however, I hate reCAPTCHA. If you are a web developer who has added reCaptcha to your site, remove it. If you are thinking about adding it in, don't do it! reCAPTCHA takes the annoying in a CAPTCHA and multiplies it by two. No longer are the days where you have to solve one illegible CAPTCHA, now you have to do two. Plus, one of them is so bad that OCR software couldn't recognize it.
It's not your or your users job to digitize books. You aren't doing the world some great justice or charity by implementing reCAPTCHA on your site. If you want to digitize books, go help develop better OCR software. If you want to stop spam, there are better, less invasive methods.