Today i found out that there is a new captcha system in Blogger which should be suitable for disabled people, but what makes me confused is that the sound produced is different with the text in Word Verification. How does it will verify when the captcha system is different? I don't know about this, but perhaps Blogger team will give the answer shortly.
Captcha system itself was designed to distinguish between humans and robots (or probably spammer) filling form or register into free services. By creating a random images or numbers which can be viewed by normal people and not to robots, we can reduce a lot of spammers in our services. You can use GD library to create image-based random number so robots couldn't identify it by reading the source.