3.8 would be great, but 4.0 would work too, I find this to be a great feature that invites people to want to reply, holds their comment in limbo if they are not registered, as soon as they do the comment appears. It's true but people just don't comment on my site (as opposed to the many many visitors we get) because of the registration process; this would be a big change and great feature if the process were reversed somehow
I truthfully don't know how they did it; aside from hiring someone to script it.