Data Usage Warning in ICS
I fixed it editing the netpolicy.xml file manually. Increased the warning limit to 4GB in 3 places as shown in the image and rebooted. Then the notification went-off:


Remount /system to make it writable (mount -o remount,rw /system)
Edit /system/netpolicy.xml file as root:

Remount /system read-only (mount -o remount,ro /system) or reboot the device