GM has already established itself as an industry authority on fuel cell vehicles, and BMW is likely hoping to leverage some of that knowledge into its own products. GM has refused to comment on the partnership, though BMW has said that the two companies are "speaking about various future technologies."
The German automaker seems to be on the equivalent of a green-tech land grab. Earlier this month, the company announced a similar partnership with Toyota designed to foster collaboration toward new green technology.
We aren't surprised to see automakers joining forces on green tech. Research and development can be costly, and it only makes sense to spread that financial burden out amongst as many collaborators as possible.