Maybe give reCAPTCHA Mailhide a try if you want to use a system with a more sophisticated Captcha system for this kind of thing: http://www.google.com/recaptcha/mailhide/
Could this instead be turned into a challenge response system.
mailhide+ would give you a disposable email, the person sending would use that, get back a captcha link, solve the captcha and your email is forwarded on. Optionally the sender is then whitelisted for sending to your address, further options would reply with your direct email address once the challenge had been completed.
This would mean that you'd have an email address to give people when asked for an email address (rather than a website).
Challenge/response emails have largely been dropped. If you implement such a system you end up on blacklists for generating backscatter. If SPF/DKIM ever become ubiquitous then we'll be able to look at using those sorts of systems again.
