A couple of weeks ago, they were saying the mild H1N1 virus isn't mutating.
People are constantly being infected with different influenza viruses, so there is an ever-present chance that the virus could undergo reassortment with one of them. There is always a chance that there could be a spontaneous mutation that confers greater virulence Same with any influenza a virus.
A couple of weeks ago, when that article was published, the sequences were very homogenous. The virus wasn't evolving rapidly. Here are the sequences, and here is a free tool that you can use to compare them. Read the directions, they're fairly self-explanatory. I haven't looked at the latest sequences from the last part of August, but until that point everything from Tokyo to New York seemed fairly similar.
The important thing to realize is that the virus does not need to mutate to exhibit increased virulence in certain hosts!.
Everybody already knew that the new influenza virus ordinarily causes mild disease, but it also has the potential to be a killer in some. Individuals differ widely in their susceptability and response to the same virus. A virus that causes mild disease in some may cause severe symptoms in others. Look at dengue, west nile, or powassan viruses. Different hosts with different histories respond very differently to the same virus. Most people have mild symptoms from west nile virus, but every now and then people get their brains melted by it. There's game-ending killer strain of west nile floating around though.
Disease is the end result of the damage caused by viral replication and the damage caused by the host's response to infection. Variation in either can alter the severity of disease. The 10-15% of people who are young, previously healthy, and coming down with severe disease are likely the victims of their own immune response. It has already been demonstrated that infection by the new virus triggers a more robust inflammatory response than infection by seasonal viruses.
The same thing is seen in infection by highly pathogenic avian influenza viruses, the SARS coronavirus, Hantaviruses, certain encephalitis viruses, hepatitis B and C, and others.