On the GameStop website, they show Halo 3 as being released around June 2006 here in the US.

Microsoft originally stated that they will release Halo 3 around the launch of the Playstation 3 and Sony stated that the PS3 should launch around mid 2006. It is possible that GameStop is basing the release date off what M$ has said about competing with the launch of the PS3.

We will keep you updated as news develops.