Once the variable has been put into a cell, you can't recover the original variable name from the cell. If you need that information you would have to save it separately somewhere (e.g., maybe in another cell array). Why do you need this?
EDIT:
Actually, there is a way if the original variable is still in the workspace and neither variable (the original or the cell copy) has changed and they are only shared with each other and nothing else (that's a lot of if's). But it involves a mex routine to look at the data pointers to see which workspace variable is shared with the cell element variable. Probably not the solution you wanted.
