I always found it amusing that in Boston, where so much of the population is from Celtic roots, nearly everyone mispronounces the word due to the name of the basketball team. Not sure how the team's name came to be pronounced that way, but I'd have to go with Faldage's theory, and figure that the average Boston hoops fan (among which I counted myself, for some time) just didn't know any better.

I once played on a soccer team in Boston known as the Celtics, and we had frequent debates about how to pronounce the name of the team.