Here in the states there are parades in the cities with large Irish populations -- Boston, New York, Chicago -- and I think even San Francisco may have a parade as well. In Chicago, they dye the Chicago River green.
Mostly in non-Irish America, it's not taken very seriously: People throw "O'-" in front of their last names for a day (I don't - O'Cafiero, just doesn't work), they wear a green article of clothing as not to be pinched or slapped (in a friendly way, I hope) for NOT wearing green on St. Patrick's Day, and Guinness is sold by the multiple boatloads.
I will try to get up to the San Francisco CSC gathering in Millbrae to watch the Bhoys. I will also be remembering Irish heroes like Tom Clarke, Sean MacDiarmada, Thomas MacDonagh, Patrick Pearse, Eamonn Ceannt, James Connolly, Joseph Plunkett, Brother Wafrid, Bernadette Devlin, Bobby Sands, and others.
And of course the immigrant to Ireland who started it all, St. Patrick.