I've been pondering making one to use as reference, but I don't hide much behind it. Of course, since I sometimes have trouble reading my own notes, my game info is usually safe.
I do prefer to hide my rolls though, simply because i've had experiences where players will assume they know what a roll means and have trouble RPing it down, and I prefer to fudge the rolls as to not kill a character in a single lucky blow. Things like spot checks, monsters hiding, things like that. I like to also make several inocuous rolls (as well as make the players do that as well) to throw them off guard.
On a side note, I've seen some interesting results with GMs who use the open die rolls. One GM nearly killed one of the most powerful NPCs in the campaign with a critical (and death is far more perminent in that campaign). With assisted magical healing, they were able to get him back on his feat in about 9 months.
Another GM had fun with me when I rolled a 1 on a search check to detect traps. After examining the door I believed I had found nearly 20 traps. So, being the greedy character I was and thinking there was a mother load of treasure beyond the door, I set about disarming them for about 20 minutes. I rolled well on the disarm and had considerable skill, and was able to disarm all the phantom traps as well as the real one (which I realized was the only one after disarming it, but I didn't have to tell the party that). He just gave me a nasty modifier to the disarm. I thought it was an interesting way to handle a horable failure.