The virus is new
Quoting a CDC (Center for Disease Control) article:

"H1N1 (referred to as "swine flu" early on) is a new influenza virus causing illness in people. ....
"Why is this new N1N1 virus sometimes called "swine flu"? This virus was originally referred to as "swine flu" because laboratory testing showed that many of the genes in this new virus were very similar to influenza viruses that normally occur in pigs in North America. But further study has shown that this new virus is very different from what normally circulates in North American pigs. It has two genes from flu viruses that normally circulate in pigs in Europe and Asia and avian genes and human genes. Scientists call this a "quadruple reassortment" virus."
Detailed analysis was done at the Canadian lab in Manitoba, and has been verified by CDC.

H1N1 virus

There are other H1N1 viruses, some similar but with different gene make-up, but this one is unique.

It has not yet been determined where this new strain originated.

Mentioned by SINSULL, a possible Fall outbreak is possible, as CDC and other laboratories suggest; detailed laboratory work shows that this strain is evolving as they watch- it could go either way, lose potency, or gain it.

CDC work shows that Mexican medical workers were quick to notice that HIN1 showed a different patient response than normal flu; medical reports praise their efforts. Thousands of tests are being carried out and it will be some time yet before accurate separation of deaths from H1N1 and normal flu is complete.