In a recent episode of Gamertag radio, Peter Moore speaks out about the increasing complexity of video game controllers, comparing today's gamepads to the simplicity of the Atari 2600's button-and-stick joystick. He didn't exclude the Xbox 360 controller's design from criticism, mentioning that his 14-year old daughter found the controller somewhat confusing. Naturally, there was a point to his self-criticism; later on in the podcast he says that Microsoft is "doing a lot of stuff there. Nothing that we're ready to talk about, and we're not going to force anything that is not going to be intuitive and innovative."
The most obvious conclusion to make from this statement would be that Microsoft is working on a new, simplified controller, which may or may not "borrow" the Wiimote's defining feature, although the part where Moore says that "[Microsoft is] not going to force anything that is not going to be... innovative" could suggest that simply slapping a motion sensor inside an Xbox 360 pad isn't on the cards.
British tech magazine T3 points out that the company could possibly be working on a simplified controller--to be sold in parallel with the main Xbox 360 pad--designed specifically for Xbox Live Arcade games. The magazine reasons that the company won't want to make the Xbox 360 pad's ergonomic design obsolete any time soon, which lends credence to the possibility of a simplified controller designed to play simple games to compliment the "hardcore" 360 gamepad.