If you use Damien Katz's Negative Captcha, you can even skirt the captcha, if your users aren't malicious: just have an invisible honeypot field called "email"

If you use Damien Katz's Negative Captcha, you can even skirt the captcha, if your users aren't malicious: just have an invisible honeypot field called "email" for the botspam to fill out, and call the visible email field something else.