No it's true. They claim it's because it would interfere with the motion sensing (the Wii doesn't have the same problem because they're using external sensors) but most people think it's just because they recently lost a lawsuit against Immersion, who sued them for patent infringment on the vibration doodad in the PS2. Immersion recently said that it is doable with the motion sensing - but Sony would have to drop their appeal against the ruling before they would tell them how.
As stated here.