pisanojm: He meant by posting multiple times. But no harm, no foul. :)
And drmike is right, I had not considered that issue. Users behind proxies (like myself, when I'm at work) can have issues with captchas. Things like HashCash and those stupid math trick plugins are equally obnoxious and annoying. They're also easily defeated.
The short of it is that captchas don't work. This is not a matter of opinion. It's proven. There are research papers and studies that have been done on the subject.
And I humbly disagree with you about spammers not using comment forms...
Also proven. Or, at least, statistically proven. Out of those 1 billion spams blocked by Akismet, I would be surprised if more than, say, 5% of them were comments instead of pingbacks/trackbacks.
And for what it's worth, I've *never* had a spam come in via the comment form. Admittedly, I've only got maybe 10,000 or so, so it's not a large enough sample to really say with any certainty. I have had a few false spam detections that came in via the comment form, mind you.