The iPhone ended the year with a blowout quarter in the U.S. market. But underneath the big numbers at the largest U.S. carriers are signs Apple Inc. faces a rapidly changing marketplace that could erode its dominance, the Wall Street Journal reports. Also, the San Jose Mercury News said that after announcing record profits – but still disappointing many analysts with this week’s lackluster holiday-quarter earnings report – Apple’s stock was dumped by so many people so quickly on Thursday that the scramble tripped a Nasdaq circuit breaker.