STANN includes a fast, parallel geometric minimum spanning tree algorithm.
Currently, it can be invoked using the header gmst.hpp, and the function

void gmst(vector<Point> &points,
           vector<pair<typename vector<Point>::size_type, 
                      typename vector<Point>::size_type> > &mst)

It receives as input a vector of points, and writes output to the
provided vector. 

Output edges are stored as STL pairs of indexes to the
original vector.  In most cases, the mst vector could be declared
simply as vector<pair<int, int> >.

Note:  This algorithm modifies the order of points in the original vector.