Hm. I don't understand many of the responses here.
I've done this as a matter of course since I started gaming years ago. I don't tell them all the stats from the beginning, but when they try something, I always tell them what DC they need to hit and why. I always roll my dice out in the open, and I always tell them what I need to roll to do what, and why, beofer I roll.
I count it as one of the best decisions I ever made when I was starting out. The amount of awkwardness and pressure that instantly goes out the window is amazing, and the atmosphere in games gets relaxed and fun, and 'game-y', which for some people might not be good, but for us it's exactly what we want.
By now I can't even imagine it doing it another way. Players rolling without knowing what they need to hit and why? Sorry, feels weird, random and boring. Me rolling some dice in secret? Sorry, feels slimy, and for that I might just as well not roll at all and just say what number I think should come up.
I would definitely recommend trying this out (to DM's who don't take their roleplaying just way too seriously, I guess).
I'd really like to hear why someone would think that 'keeping stuff secret' is the key to a better game (and something like an actual explanation instead of just a vague 'it's superior' attitude is what I'm looking for).