32% of Q1 U.S. smartphone sales involved prepaid phones, up from 21% a year ago, says NPD. That trend could spell trouble for AT&T (T) and Verizon Wireless (VZ, VOD), whose recent growth has been heavily tied to selling costly postpaid data plans. With 72% and 61% of the carriers' postpaid bases now respectively on smartphones, they face pressure to gain prepaid buyers without cannibalizing postpaid subs. Cannibalization fears have led AT&T and Verizon to make their prepaid plans 3G-only; Sprint (S) and T-Mobile (TMUS) offer 4G. A mix shift towards unsubsidized prepaid phones could also be a challenge for Apple (AAPL), estimated to have just 8% of the Q1 U.S. prepaid smartphone market (though that's up 4x Y/Y).