(I edited more into my previous post, btw.)
When I first started gaming on the NES, yeah, I had to learn how to press a button. When I started playing on the SNES, yeah, I had to learn how to manage shoulder buttons. When I got a DS, yeah, I had to master stylus-based control. And when I started playing on the Wii four years ago, yeah, I had to learn how to manage motion controls. Today, every single one of these is second nature. So no, motion controls do not inherently have some learning curve that any other input method, when first encountered, did not.