The other development possibility I have heard, is that the airport contiues as an airport til 2005, then most of the land which must be rather valuable now is sold off for business development.
The airport's remaining buildings and apron becomes a city heliport.
At least some aviation activity will be kept close to the city.
It also makes sense, as before the airport was scaled down it put out information that it was wanting to attract more heli business and refered to itself as a heliport for Sheffield in the press. At the moment the regular business of the airport comes from the police helicopter, a helicopter flying school/air charter company, and I believe at a later date an air ambulance is to be based there.