And a final follow-up: Everything working as expected, with a green light from the W3C validators once more. After all the time I spent researching and fixing things like the regurgitated Facebook code to validate, this was really bugging me *laugh* Thanks again, Marcher.
Wonderful hook. I had two or three spam registrations making it through the regular service per day, and every one of them was showing up in the stopforumspam database with an extensive history. Installed this hook, and not one spam registration has made it through since. Spending a lot less time researching, banning, and deleting spammers, and a lot more working on my site.