Everything you need to know about linear and radial ball bearings!
http://www.youtube.com/watch?v=5Dkruh8NO9E
@Mark_Rehorst what kind of belts and pulleys are you using?
It all points to a belt issue - either the pulley is not up to par (tried swapping X and Y?) or the belt is too weak or too stiff, which might get the entire spring-mass-spring-mass system that is the motor, belt and bed into resonance. I use two 15mm wide, 3mm pitch HTD belts (and two NEMA17 motors) for my 440x220mm bed, which carries a 6mm G10/FR4 base, a 4mm aluminum bed and a couple of heaters.
But it might also be something entirely different. Maybe post a video in the G+ 3D printing community and see if they can think of something else?