Practical non-image based CAPTCHA approaches?

Question

My favourite CAPTCHA ever:

A method that I have developed and which seems to work perfectly (although I probably don't get as much comment spam as you), is to have a hidden field and fill it with a bogus value e.g.:

<input type="hidden" name="antispam" value="lalalala" />

I then have a piece of JavaScript which updates the value every second with the number of seconds the page has been loaded for:

var antiSpam = function() {
        if (document.getElementById("antiSpam")) {
                a = document.getElementById("antiSpam");
                if (isNaN(a.value) == true) {
                        a.value = 0;
                } else {
                        a.value = parseInt(a.value) + 1;
                }
        }
        setTimeout("antiSpam()", 1000);
}

antiSpam();

Then when the form is submitted, If the antispam value is still "lalalala", then I mark it as spam. If the antispam value is an integer, I check to see if it is above something like 10 (seconds). If it's below 10, I mark it as spam, if it's 10 or more, I let it through.

If AntiSpam = A Integer
    If AntiSpam >= 10
        Comment = Approved
    Else
        Comment = Spam
Else
    Comment = Spam

The theory being that:

• A spam bot will not support JavaScript and will submit what it sees
• If the bot does support JavaScript it will submit the form instantly
• The commenter has at least read some of the page before posting

The downside to this method is that it requires JavaScript, and if you don't have JavaScript enabled, your comment will be marked as spam, however, I do review comments marked as spam, so this is not a problem.

Response to comments

@MrAnalogy: The server side approach sounds quite a good idea and is exactly the same as doing it in JavaScript. Good Call.

@AviD: I'm aware that this method is prone to direct attacks as I've mentioned on my blog. However, it will defend against your average spam bot which blindly submits rubbish to any form it can find.

Unless I'm missing something, what's wrong with using reCAPTCHA as all the work is done externally.

Just a thought.

The advantage of this approach is that, for most people, the CAPTCHA won't ever be visible!

I like this idea, is there not any way we can just hook into the rep system? I mean, anyone with say +100 rep is likely to be a human. So if they have rep, you need not even bother doing ANYTHING in terms of CAPTCHA.

Then, if they are not, then send it, I'm sure it wont take that many posts to get to 100 and the community will instantly dive on anyone seem to be spamming with offensive tags, why not add a "report spam" link that downmods by 200? Get 3 of those, spambot achievement unlocked, bye bye ;)

EDIT: I should also add, I like the math idea for the non-image CAPTCHA. Or perhaps a simple riddle-type-thing. May make posting even more interesting ^_^

What about a honeypot captcha?