Find the distance from each node to every other node, and sort() the result recording the indices (the two-output version of sort). Be careful about the distance of nodes to themselves.
Yes, there are probably more efficient methods. Or at least methods that use less memory. In particular... http://www.mathworks.com/help/matlab/ref/voronoi.html -- though at the moment I do not see any convenient routine to determine which voronoi cell a particular point is in.
