Google will block searches if their systems detect something is up. "Up" is often an infected computer that has a virus trying to looking for email addresses or bbs vulnerabilities via Google searches.
Increasingly, though, "up" might also be companies that naturally do an awful lot of Google searches by hand and rather than block these people outright Google challenges them with a captcha so they can prove they're not a virus.
... but what about in China? You ever seen a Chinese character captcha? Just how would that work?
Pretty cool, huh? The captcha is numeric and there's a lovely calculator-esq type pad to use.
Google recommends this anti-virus to their Chinese users. They point to this do not crawl us help page as another explanation as to why you're seeing the captcha page.