How dare there only be 2 mentions of Nier in this topic. Sure, its not a PS3 exclusive, but its really awesome (if depressing at times). Plus New Game + adds more to the story, making it even more depressing! Good game, although I don't recall it being particularly difficult (can't remember if it had multiple difficulty modes though).
Everyone has pretty much talked about Tales (Never played any sadly, always wanted to), Ni No Kuni (Looked a bit generic/cliche to me, not that that's a necessarily bad trait), and Valkyria Chronicles (I loved this a lot) which seem like good picks. I'd recommend Final Fantasy 13, but that's a real love it or hate it type of game, although many like its sequel apparently (I personally found it to be pretty average, while I found 13 to be above average). Guessing you might have played this on the 360 or whatnot.
White Knight Chronicles is supposed to be at least decent, and if you buy the sequel, it comes with the first game as well. I only tried it once for about 20 minutes and didn't like the combat, but it looked interesting. You also have Demons/Dark Souls, but I've heard the difficulty in those can be quite up there. I've only played a bit of Demons Souls, and while I found combat to be annoying, it was also rewarding.
If you haven't played Kingdom Hearts or Final Fantasy X, those are going to be remastered in HD for PS3. Not exactly new or anything, but they are RPGs at least. Kingdom Hearts is supposed to come out fall of this year, so that's pretty close.
But seriously, go get Nier, and play it at least twice to get the full experience. I really love it, and feel its an underrated gem. =(