Bill,
The corn and wheat innoculants are slightly different than the soybean and alfalfa innoculants.  As you know, the alfalfa and soybean innoculants contain rhizobia which produce nitrogen when they form that symbiotic relationship with their hosts.  The corn and wheat innoculants contain fungal spores (not rhizobia) which also form a symbiotic relationship with their host but do not produce nitrogen.  Instead, they colonize within the rhizosphere of the roots and defend against many different root diseases.  They are also root growth promoter as well as increasing stalk diameter in corn.  The healthier root system is able to scavange more nutrients as well as take up more water.  That is the short version on how they work.  There is a lot going on in the rhizosphere!!!!