I think I am with belfast on this one. I find the word to be contextual within a given conversation. Jim Duffy, an ex pat from Derry and a very close friend, will always start a conversation with "What's the crack, Mick?". Several other expat friends of mine from Tipperary use it in the context mentioned above "Did you hear the crack about........?". All of the Irish ex pat's without regard to where they are from, use it to describe a grand time had by all as in "The crack was grand at Billy Quinn's party.....". As to how old it is, I have no doubt that it is old, but I can tell you that I never recall it being used by the old immigrant crowd of friends of my grandparents. It seems that it wasn't until I was well into a group of friends of my own age that I heard it on a regular basis. As to spelling it, I find the "craic" spelling to be easiest when I am writing for the "casual" Irish American, as it eliminates any confusion, but I usually point out that it is not a word from Irish.

All the best,

Mick