Just saw this one.
Star Wars Episode III:
Insert disk 1
When the main menu comes up goto "Options"
While in the Options menu. on your remote press "11" then select/enter, "3" then select/enter, "8" then select/enter.
A video will start showing Yoda dancing and rapping.
Enjoy
Edited to add: You can also use this same thing to access the blooper reels on episodes I & II.