The real danger from the swine flu is not H1N1 (which most people have some pre-existing immunity from other flu strains).
The antigenic shift originating from a pig or bird co-infected with H5N1 (bird type influenza) and H1N1 (swine type) to a highly virulent form that is freely transmitted between humans is the big whammy.
Shorter version: When the piggies get bird flu, watch out.