Study Shows Swine Flu Potential

Scientific American:

Choi wanted to assess the pandemic potential of Korean strains. His team tested two H1N2 and two H3N2 viruses isolated from pig abattoirs before the 2009 pandemic. Most of these viruses did not cause any signs of serious disease in ferrets.
Sw/1204 was the exception. It replicated in the airways and lungs of three infected ferrets, killing one and causing such severe disease in the others that they had to be euthanized. The virus also spread through the air to infect three healthy ferrets that were housed in cages next to infected ones.
The virus gained two new mutations in its trip between the cages — one from aspartic acid to glycine in the haemagglutinin protein (HA225G), and one from serine to asparagine in neuraminidase (NA315N). The mutant virus was better at infecting and growing in human lung tissues and airway cells than the parental strain, and could still thrive and spread among swine.
The HA225G mutation allows the virus to bind more effectively to receptors in the lungs of its hosts, and has been linked to greater pathogenicity, but not transmissibility, in the pandemic H1N1 strain. NA315N has also been found before, but its role is less clear. Choi suggests that it stabilizes the neuraminidase protein, which is involved in viral break-out from host cells.

The outbreak of swine flu in people at county fairs is slightly concerning.  At least it hasn't shown any potential for transmitting from human to human.