There will be two answers on the complex plane. I have not worked them out yet.
This is my initial attempt to make sense of it:
√−i=√−1∗√i=i∗√i=i32=(√−1)32=((−1)12)32=(−1)34
But wolframalpha says that one of the answer presentations is −(−1)34
http://www.wolframalpha.com/input/?i=sqrt(-i)
So even my inital attempt is not correct I do not where that negative sign out the front came from.