I use the timestamp method Martijn is suggestion for all my contact forms. But what I do is check that the timestamp isn't to far in the past. I assume some spam bots crawl the form one time and then repeatedly send different body's with the same hidden values attached. That way the bot would have to re-crawl the form every 5 minutes, or be unable to send any more spam after that period. The prevent really smart, or human guided bot from detecting the timestamp and changing it to the current time, I (also) add a salted hash of the timestamp. When the form is posted I check that the salted hash of the send timestamp matches the send hash. If it matches I check if the timestamp is recent. If so I accept the message.

I hope someone finds this method useful, because I hate people just throwing captcha's around. Like captcha's are the only solution!