I had a 3G and the vibration was not all that. The iPhone4 seems pretty similar to me. THe funny thing is someone could call me and I would not feel it when it was "ringing" yet I could feel the voicemail alert.
I also noticed with the 3g (which I had to return for new phones a couple of times due to massive dust infiltration) that some of them had stronger vibrations than others and also that some were louder when vibrating. It seemd that the louder ones (2) were not as easily felt and the quieter one seemed stronger. I guess that is consistent with the motor not being well coupled to the case.