Reply #21: From the link in the OP regarding immunity:

[dflprincess]

No one has complete immunity to this strain but most of us have been exposed to A strains of influenza in the past and those of us who are old enough have not only been exposed but probably had an A strain variant (I know I have, the last time was enough to convince me that flu shots are wonderful things).

http://www.wlwt.com/health/19294743/detail.html

It's possible that people who have been exposed to flu viruses every year -- especially older people, with a greater exposure history -- may have some natural immunity, the CDC official said in a call with reporters.

also:

http://www.timesonline.co.uk/tol/news/uk/health/article6175909.ece

H1N1 flu strains similar to the one identified in Mexico circulate seasonally, and most people have some residual immunity to them. But in a process known as antigenic drift, strains of the flu virus undergo genetic changes that make it less familiar to the human immune system, even unrecognisable....

...it appears so far that only severe cases are being picked up — what we don’t know is the background of mild illnesses that are not being reported or identified. It is also a good sign that the virus appears to be vulnerable to antiviral drugs that are already stockpiled in Britain.