At what point do items that alter specific skills start appearing?

I have heard there are equipment attributes that will modify specific skills, such as "decrease cool down on wall of zombies by 2 seconds".

I'm really looking forward to these, as it may open up some new build strategies.

At what point might I start seeing items like this drop?

Though these items are fairly rare, I began seeing them drop around the end of Nightmare, and with increasing frequency through Hell and Inferno (though I can't say I've seen any that were any good yet). Expect rare items with good skill modifiers to be extremely expensive as a result - you have to be pretty lucky to get a good skill modifier in the first place, and a lot of the ones I've seen have been on otherwise trash uncommons.