EyeToy: Groove:
Master level difficulty:
To unlock as master difficulty, get an A in one of the tracks. You will then have a new difficulty level. It is (as far as I can tell) exactly like hard mode but with less warning when the heads will appear.
Track 21-25
Complete tracks 1-20 all the way through on any mode.
Unlock Songs 23-28 (NTSC Version):
Complete the first 23 songs on any difficulty.