Rather than repost the history of the holiday in this thread, I'll refer you to the Irish food question thread, where I just posted a history of the U.S. holiday from the U.S. cable History Channel.

Heathens stealing the holiday away from the church and sports types in Ireland, and celebrating it "American style" is the best thing that ever happened to the holiday. It is now quite the party scene in Dublin, which seems much truer to our Irish identity. We even allow gay marchers in our parade.

As to the "since when did Birmingham and London have St Patrick's day parades" question, all I have to say is it is about time the Irish in Britain got on board.

The best aspect to me of the celebrations in the US is the inclusiveness of the day. Everyone is Irish on St Paddy's Day is a wonderful sentiment, IMO, and one of the reasons why Irish citizens are more warmly received around the world than we deserve to be, in many instances. We have the Yanks to thank for that.