But Google really did not hide all these advertisers from their publishers. It appears that based on how you actually access the Ad Review Center, you may see different options.
Russ noticed it seems to be a rendered bug of some sort. Russ had to click on the blocked tab and then re-click on the allowed tab. Once he did that, the ads showed up again.
I tried this myself and it is 100% a bug. You load up the Ad Review Center and the "Allowed" tab shows up empty. So I clicked on the "Blocked" tab and then clicked back onto the "Allowed" tab and the ads showed up again.
So this seems like a small bug that seems to be freaking out some AdSense publishers.
Forum discussion at WebmasterWorld.