This all probably has something to do with the new XRumer being out. A bunch of people have cracked recaptcha before and my guess is it's now standard in XRumer.
As far as i have read, its because of Xrumer 7 which is spreading the spam with ease. I added two custom profile fields with drop down answers on registration, and bot spam registrations are story of past.
I've given up on a few forums in the past because I just couldn't cope with all the spam, use to drive me mad having to delete 80+ registrations daily. A forum I am currently Admin on is getting bad as well, although we have done a lot to clear it up recently.
Another alternative solution to prevent spam on your site is to use a hidden timer/script that will moniter the over-all timing for the registration process: filling fields and submitting. This way you could simply seperate humans from spam bots, cause bots knows the fields names and auto-fill them accordenly - then fill the captcha and hit submit.. quite fast Wish I could help you with that - or post a code/plugin for everyone, but I'm quite busy - Why don't you PM me sometime after the weekend? Might sort something out for you. Bet you would do much better than him btw better start now.
Search "decaptcher" on google and there will be quite a few results. They're mostly quite cheap for bots to use for spamming, I believe that some of them are bots that take educated guesses at the captcha, and some are just people that sit at the computer typing captchas all day.
I found an easy to keep these SPAM/BOTS out of my page..there is a company called Confident Technologies and they are using Imaged-Based Authentication. Had it installed for about a week and I am very STOKED!!!!