Brake pad build up can cause a similar symptom as warped rotors. The residue doesn't build up perfectly even, and also will slip-grab-slip-grab causing a pulsing/vibration - a similar symptom of if the pads are just glazed over which is possible.
If the rotors are not warped then new pads, brake cleaner, and a Scotch-brite pad would be step one, and bead blasting the rotors would be great, if possible.
Headstock bearings, as mentioned, are also another source of mysterious vibration under a load.
Last edited by Cocky Rooster; 09-28-2010 at 11:14 PM.