Ford,of course, is one of the top sellers of light trucks (F150) and body on frame SUVs (Expedition, Navigator). Toyota is not a major player in trucks, but does sell Tundra and a variety of BOF SUVs such as the Sequoia and 4Runner. By working together, the two companies can reduce the high development costs of a new hybrid powertrain, and eventually be able to meet or exceed the CAFE standards for light trucks.
What is odd, to me, about this tie-up is the cooperation between to major rivals. Remember recently the dust-up over IP, when Ford and Toyota agreed to cross-license HEV patents, and the ensuing confusion in the consumers mind, where some people thought Ford was buying Toyota's Prius system. Why not GM? GM/Allison have been doing RWD truck hybrids for a while now. Oh, maybe this is why not.
In general, we are going to see more of these cooperative moves. For example, Chrysler will need to get in bed with someone to develop their own truck hybrid system, as trucks are a big part of Chrysler's business model.