We've asked for a CAPTCHA ever since botting became a problem in Diablo 2. Not to be a cynic but I doubt we will ever see Blizzard utilize one in game. Besides I don't really think a CAPTCHA system will be all that effective. Sure it will slow them down but they will then just get a guy to type in the CAPTCHAs when he sees them. You can't stop criminals, you can only slow them down.
A much better solution to this problem is as Overneathe suggested...
Implement user-created chat channels where the creator of the channel can ban other people.
Also implement custom games with host/vote kicking/banning. Hell, even just a regular old vote kick system in match making would help a lot!
I wouldn't mind CAPTCHA, but sometimes those "words" are impossible to read. You waste more time at refreshing new captcha instead of playing the actual game:P
And man, I'd love one of those D2 action figures
Entering a captcha has become a normal step on the internet, from creating accounts to downloading anything. I would not be against such a system if it worked and didn't pop up mid battle. But i agree with the blue, if such a system was implemented a lot of people would revolt quickly against it.
I'd rather they do banwaves every week and not with 2 month delays instead of captcha. The last wave was extremely effective but they can still bot 24/7 for the whole month or two before the next wave comes.