I think I have felt it once or twice. Sitting at a light, just after driving a few blocks after a cold start. Nothing major. It only happened for a second or two. After I left that light, the car warmed up to normal operating temp, it went away.

I'm thinking it could be engine vibration and since the motor mounts aren't very stiff, it probably starts to go into a small dance. The vibrations could be just right that it causes the motor to disturb the balance of the car for a second.

Just a thought. I personally am not going to worry about it. I wouldn't if I were you, unless you heard some strange noises associated with it. =)