According to Yahoo News:
The Sun newspaper claimed that the split was caused by the huge pressures on the young couple and because of the time William has devoted to Army life.
A "close friend" of the couple told the paper: "As far as Kate is concerned, William simply hasn't been paying her enough attention.
"She is stuck in London while he is living in an officer's mess in Dorset. Kate feels hugely frustrated that their relationship just seems to be going backwards at a rate of knots." Continue reading.
Personally I don't see this lasting, and I reckon they'll get back together again. That is when homegirl's girlfriends get finished talking some sense into her, "Are you bloody mad? You stand to be the future queen of England. We think you can put up with the occasional lack of attention. And what William doesn't give to you, you'll get more than enough of from the paparazzi!"
Or maybe it was the thought of spending holidays at Balmoral with Prince Phillip banging on - that's enough to put anyone off.