I know that MacBook Pro has shock sensor installed in the laptop, but just got stunned that my MacBook has it too (maybe I haven't learned all the features so far). This morning I found an small application that can detect and simulate MacBook orientation. LiquidMac is a freeware which requires you to have a MacBook, MacBook Pro or latest PowerBook, and of course Mac OS X 10.4.
After downloading it (it is just 98K), I just launched the application. Then by gently tilt the MacBook the liquid animation will react as if I tilt a jug of water. There are some parameters to set to get different effect, but the default should be stunning enough.