i have followed this steps:
1.Add the AudioToolbox framework to your target.

2. In the file you intend to trigger a vibration, import the AudioToolbox header file:
#import <AudioToolbox/AudioToolbox.h>

3. Finally, call the following line to make the device vibrate:
AudioServicesPlaySystemSound(kSystemSoundID_Vibrate);

nothing happen in my iphone4...
ths system volume is at max volume...
there is some settings that inhibits this function?

thanks