I have literally heard kids shout out "Oh! That's why we learned those x y things in school!" when learning how to move sprites about on a grid. They have no idea they are learning math, and they generally have a spectacular time making even the simplest game, like a scrolling shooter, or moon lander.
My favorite discovery during our three years has been seeing how kids name their games. It's never "David's Moon Lander." It's always something more like "David's Super Awesome Cat Launching Space Lander With Flame Throwers." It's like watching kids draw anything they want with crayons: they get to theme and tweak games however they see fit.
As a volunteer organization, we can always use more teachers to help us spread Scratch to the masses. Our classes are free and offered to kids ages 8 to 14, and we do a simple game each week.