I believe you are most contagious for about 48 hours BEFORE you show any symptoms. In other words, you are most likely to pass it along to others BEFORE you even feel sick, which is one reason this spreads so fast.
My best friend works at a hospital and got H1N1 earlier this year. Her first symptoms were a really bad sore throat and stuffy head. She thought she had strep throat, but they tested her for H1N1 too and that was what she had! Although she must have been contagious for a couple of days before that, no one else at her office got sick. They sent her home for 7 full days until she was totally over it- if you get it or suspect you have it, do NOT go back to school or work early. Make sure you are totally over it and immune before you return!
She said it was just like a really bad case of stomach flu, only with a bad sore throat. They put her on Tamilflu and she felt much better in a couple of days, but was not allowed back to work for a full week.