From [1]:

The deadly H1N1 influenza virus that’s fueling fears of a global pandemic is a hybrid of two common pig flu strains [North American and Eurasian pig flu], scientists who have studied the disease told Wired.com Tuesday. Earlier reports called it a combination of pig, human and avian influenza strains...

"The new neuraminidase gene that came in from Eurasian swine is one we’ve never before seen circulating in humans," said [Andrew Rambaut, a University of Edinburgh viral geneticist]. "That’s one of the reasons it’s spreading rapidly. Very few people will have any immunity to this particular combination, which is what gives the concern that this will be a pandemic rather than just a normal seasonal flu outbreak. It remains to be seen how much and to what extent there is existing immunity."

[1] wired · com/wiredscience/2009/04/swinefluupdate