Aweber does use captcha forms now and sometimes right after you opt-in you will be redirected to one.
I first noticed it myself about a month or so ago when I looked at a subscriber notification I had gotten and saw the long link in the place where the squeeze page link that the subscriber signed up from normally is. I clicked on it and it brought me to a captcha form.
I don't know what triggers these. It might be random, or maybe it triggers on email addresses that have been used to opt-in to lots of Aweber forms, or maybe it's based on IP. I really don't know why but I know they are in fact using captchas now.