I ran across a site called scr.im (http://scr.im/) today. In my personal opinion, it is an extent of url shorten (e.g: http://bit.ly/).

You basically enter your email address and it will generate a shorten url for you with a simple mechanism to force users to choose the matched text block, a simplified version of captcha.

Looking to see this in action, give mine a try: http://scr.im/jayzeng

When clicking the text block, a POST request is made:
captcha=E3M94&action=view&token=87735429e36a2256b62b5ed27de9eeda&ajax=y

Obviously, captcha is the name and value of the captcha text, token appears to be a text-based (possibly) md5 hash (since it is 32bit and alphnumeric). My bet it is

md5(current time + salt). 

My quick attempt is it does not implement any

if(number of failures > predefined_number_attempts) {
  block_ip() OR user_has_to_wait(5mins)(); 
}

This introduces the opportunity for attackers to brute force the site, which makes this service absolutely no-sense.

Well, not exactly right, it seems to me this site is simply an attempt to explore a new way to reduce number of spam, with little effort (and added complexity for end users).
Most users for this site will be individuals and/or small businesses. So even if the site is breakable, any one really cares? It is only an email address that is intended to share with a group of interested parties.