If you have a 360, hop on the marketplace -- a trailer for Halo 3 has surfaced as a free download, in addition to trailers from several other games, including Splinter Cell. From the description on the XBL Marketplace:

With its defensive forces all but obliterated, Earth has fallen to the unstoppable might of the Covenant. These alien occupiers have discovered something be need the ancient African sands -- something incredible.. something Forerunner.

That is all.