Am running current version (2.3.2) and have received two disturbing comments about an inability to register and make a comment to my blog jcwinnie.biz/wordpress
Under Options -> General the box allowing membership is checked.
The feedback from thwarted commentors:
1) I've been trying to leave a comment on your AG site, but for some reason WordPress won't let me register.
2) I get annoying messages when I try to (and it won't let me register again as a new user either)...
You might get some comments if WordPress wasn't so hostile to intermittent commenters who can't remember their password :-)