Seeing as Cider has basically done everything that can be done with current content (at least, judging from his videos ), he's probably just experimenting with different specs. He might even be Arms/Prot because he doesn't need the mitigation in current content. I wouldn't put too much emphasis on his spec these days - wait till Ulduar
In any case, Cider's current build is a single-target high-threat beast. You definitely wouldn't want it for general purpose tanking, but from what I hear (haven't tried it myself) the UA build has the highest TPS/DPS on boss fights of any Warrior tanking spec.