You ask very good questions. There have no doubt been a significant number of asymptomatic infections and infections so mild that the person might have totally ignored their symptoms as insignificant (i.e. subclinical infections). For instance, it's possible that someone might have had nothing more than a slight runny nose for a few days and thought nothing of it...thinking it just seasonal allergies or something like that.
Asymptomatic infections have happened in the past with other influenza strains and that's well-documented, so this isn't something new to pH1N1, but nobody really knows exactly how many people might have had asymptomatic or subclinical infections without testing for antibodies to the virus and taking medical histories. One way we often find these asymptomatic prior infections is when a group of healthy people volunteers for a research protocol, like taking a new vaccine or testing a new medicine, and during the intake process a sample of their blood gets drawn and some of them are found to already have antibodies to the virus yet didn't report ever having been sick with it or having been previously vaccinated against it.
If someone catches flu, doesn't that necessarily mean they'll have some symptoms? If the virus is replicating, wouldn't it always cause noticeable symptoms?
It is possible to have been exposed to a virus in a dose low enough that you don't have any symptoms. For each virus, there is a certain minimal exposure necessary to produce clinical illness. The exposure dosage needed to produce clinical disease varies from virus to virus and has been studied by scientists. You can do a search using the terms 'HID50' (Human Infectious Dose 50 = the human infectious dose of virus needed for 50% of people exposed to develop clinical disease) and 'influenza' if you want to know about the details for flu. What most people consider "catching the flu" is that you come down with the classic symptoms of flu, that you are sick. You aren't considered to have "caught the flu" (at least in the vernacular) if you were exposed to the flu virions but never got sick. In medicine, we use a distinction: colonization versus infection. You are said to be colonized with a pathogenic microorganism if that organism grows in your tissues without causing any signs or symptoms of illness. You are said to be infected with the pathogen if it causes tissue damage and illness. As an example, many people have staphylococcus bacteria on their skin and in their noses. They don't have staph infections. However, if that bacteria invades the mucosa and causes inflammation, redness, pain, etc., then the person is said to have a staph infection.
Hope that explains it somewhat!