EB Games has a pre-order of Halo 3 up for $59.99 USD stating that it will ship in June 30th, 2006 though they put in big bold letters:
The ship date and retail price have not been confirmed and therefore are subject to change. If the retail price is decreased you will receive the lower price.
So we still don’t know when to expect it or really how much it costs, but you can put your name in for the pre-order.