The CDC's medical journal Emerging Infectious Diseases has just published the report on that first feline H1N1 case from 2009 in Iowa. This was a big deal, as evidenced by the FIFTEEN co-authors, people involved in the case who really really really wanted their names attached to it!
The take-home message for cat owners is, firstly, that cats CAN catch H1N1 flu from people and, secondly, that the presenting symptoms in this case were depression, decreased appetite, and difficulty breathing (as evidenced by reluctance to lie on its side, sternal recumbency, and stretching the neck out to increase tracheal airflow). The cat did not have sneezing/nasal congestion. In cats, H1N1 causes lower airway disease (lungs) rather than upper airway disease (nose, nasopharynx).
The illness resolved with supportive care (fluid administration), good nursing (nutritional assistance), and antibiotics as a precaution against secondary bacterial infection.
We still don't know how common H1N1 is in cats. That report will no doubt be published sometime this year as statistical data over time gets compiled. If you get sick with fever, aches, etc (flu-like symptoms) and then your cat develops lethargy, depression, or decreased appetite, please bring it in as a precaution. Do not try to determine on your own whether or not there is lung involvement.
It helps to bear in mind that the vast majority of respiratory problems we see in cats sre mild and easily treated upper respiratory infections due to Feline Herpesvirus. Pneumonia, asthma, and more serious problems are in a distinct minority.