LittleGreenFootballs (a semi-political blog) has used random account creation windows for the last several years, to my understanding. It seems to heighten urgency, create a sense of exclusivity in the blog commenting community, and achieved the original goal of putting a big barrier in front of trolling the site. (You have to keep an eye on it for weeks, snag an account, THEN troll.)

I don't think I'd ever implement this myself, though.