It seems to me that Q&A is the answer, if done properly. The key is to ask something that can only be answered if you're on the site. For example: "Next to the Slashdot logo at the top-left of the page, there is a five-word phrase. What is the second word in that phrase?"
You'd obviously need to change it up fairly often (and large sites would have problems still), but spammers would have a difficult time keeping track of answers for thousands of sites.
To make it even better, have it rotate through a few similar questions for your site, and have the questions be buried CAPTCHA-style in an image.
All told, it would seem to help. They'd have to resolve a very long CAPTCHA (117 characters in my example above) AND be on the site to get the answer. Seems like it would help.