I'd happily take this over mass obnoxious in-your-face spamming any day.
I'm relieved that the comments on this are mostly against it. I was sort of worried. A captcha is a very delicate balance between not pissing off the good users and keeping the bad users out.
And let's not forget what seems to be the most effective method of cracking a captcha, to just proxy it to an actual user who thinks they are verifying themselves for some other site (porn). This doesn't address that at all.