Stone,

You may want to check your timing. I just had mine checked and the timing was way too advanced (by about 4 degrees). Once the timing was adjusted to factory spec, the pinging was almost completely gone. I still get a bit of pinging under really heavy acceleration, but it's not as much as before.

I agree, buying premium fuel just to get rid of pinging is too expensive. Give the timing thing a try. It just might do the trick.