It looks like those are HEV's being dropped prior to the New Mombassa Slipspace Event. Most of the pods are caught in the blast, but one lands.... I wonder what this could be? Clearly it is for Halo 3, as it states in the trailer.
*Starts talking in Robert Stack Voice*
There is alot of erieness to this trailer. Only time and patience will discover what it really means. Hopefully, we will learn more about this mysterious occurance.

You know what the worst thing about eating a hot pocket is? Getting burned by the magma-cheese. :'(