Not sure about the US, but here in Canada, it's really just a bit of fun. Folks wear green (if they don't want to get pinched), beer and other drinks get colored green, special food hits the menus (ahhh, the truly magical McDonald's Shamrock Shake!), and we hear bad accents attempted all over the place.

I think it's as much about an excuse to have some fun and welcome the coming of spring and going of winter than anything else. I have never gotten the impression that anyone actually takes it seriously.