I've mentioned this a few times over the past couple of months. My logic being that they want to build up Ryback as a credible main eventer, as Vince has been quoted as saying he thinks he could be the #2 babyface in the company. And the fact that Brock is leaving after Wrestlemania (if he doesn't sign again) so I figured Brock losing to Ryback might be a great way for him to go while putting over a younger guy in the process. Ryback could lose and still come out looking great, but it wouldn't be too soon for him to beat Brock either. There's still plenty of time before Wrestlemania for Ryback to get more significant wins under his belt (against Big Show? Mark Henry? Etc.) and as I recall, they did pretty much the same thing with Brock himself during his first run in 2002. He beat only midcarders, then after only four months of that, he all of a sudden beats The Rock (of all people) and becomes WWE Champion and a made man, and no one complained about that. It established Brock as a main eventer right then and there.
That said, I don't see the match happening. Most likely, you'll see Rock-Cena-Punk and Taker-Brock, which isn't a bad thing at all. But Ryback against Brock would still do well to put over Ryback, and it's not at all implausible that they would do this route.