<p>With text-recognition improving and CAPTCHA-breakers using Mechanical Turks to break otherwise unbreakable keys, what's the next technology to keep scripts from spam-botting a site that relies on user input?</p>

I am a fan of limiting logins by using a credit card or cell phone SMS (like Craigslist and Gmail). These methods don't cost much (<$1), but can be highly effective in keeping spam accounts under control.
However, this is tricky on a site like SO because one of the founding goals is to have minimum friction and allow anonymous users to contribute. I guess that's where the throttling and voting comes into play.