MovableType 3.x allows you to restrict commenting to those authorized by SixApart's TypeKey. After turning it on, I noticed that I was not only not able to comment (on my own entries), but getting the following unhelpful error:
TypeKey has encountered an error. The requested page could not be found or the requested action could not be completed. Please check the URL carefully and try again. If you accessed this page from an email, make sure you cut and pasted the URL accurately.
This was resolved with some sleuthing -- special thanks to Paul Philippov. See answer in MovableType Knowledge Base. Log into TypeKey and scroll down to the bottom of Account Preferences. Under Weblog Preferences you should find a link to the blog URL in question. You must end with a trailing slash, i.e., "http://torque.gig8.com/". Lame.