The best advice I've both heard and passed on, when asked about hiring "data scientists", is you want someone who can look at raw data, massage it, and develop their own original opinions and insights about it, preferably derived from a deep understanding of the nuances of statistics and probability and the messy real world. It's the judgement and insight you're primarily looking for. So, hire the best mathematician, statistician, probabilist, economist, or heck biologist, epidemiologist, etc. you can find, and then teach them the vocational tools - hadoop, etc.