I know the plugin says it may not work as of 3.1.1, but I successfully implemented it on a development site that uses 3.3.1. I'm running multisite with a primary site (public) and a members only site (login required). I downloaded the latest version of the plugin today.
On the development site, I network enabled the plugin, used the same settings on both sites, and it worked beautifully.
I've been working on launching the site today, and the only difference is that on the production site, I'm running 3.3.2. I network enabled the plugin, and with my custom settings, it works great on the public site (users must be logged in to comment).
Here's the weird part. When "Activate" is unchecked, I get the default CUSTOM LOGIN page (with the gray diagonal striped background). When "Activate" is checked and my settings are entered, I get the DEFAULT WORDPRESS login.
How backwards is that?
I'm using Genesis 1.8.1 and a child theme. I'd post links, but I'm still tinkering, so they may not be representative.
Incidentally, I tried the A5 contact form too, and it also didn't work, so I don't think this is a problem with the plugin necessarily. It's just strange that it works in dev and not in prod. Any advice is appreciated.