It appears that the ReCAPTCHA plugin that I was using to verify user registrations has been broken for some time now. I'm not sure exactly how long, but looking at my backups of the site pre-Lenny upgrade it doesn't seem that it was properly installed even there. I'm not sure how this happened, but it should be fixed now (and based on the spammer who registered this morning, I feel fairly confident saying that registration is working again. ;-). My apologies if this prevented anyone from registering.