It's only Random House and its subsidiaries. So now all 6 of the biggies are part of the Agency Model. Don't worry, it just happened. It was a rather sudden announcement in the last few days by Random House. I think it was only brought up here a couple days ago but I just looked on MobileReads and I think it was mentioned there a day before that, so RH didn't really give much notice and it went into effect today.
Random House only announced it yesterday afternoon. BearLion was the first to catch it and post about it: http://bookclubs.barnesandnoble.com/t5/NOOK-General-Discussion/Random-House-to-Adopt-Agency-Model-Starting-3-1/td-p/883404
As soon as I found out, I updated the $5 NOOKbook Sale posting. But it was really short notice from RH.
Yeah, I got caught off guard by that, too. I knew the $5 price was potentially for a limited time, but I wasn't aware it would affect other, older books by the sam publisher. I saw quite a number of books on my wish list go up in price, not just the ones on the $5 list. I only blame myself for procrastinating, though