There appears to be a misconception here. Mahalanobis distance has no meaning between two multiple-element vectors. Ideally it is a distance between a vector (or distances between a set of vectors) and some given distribution defined by means and covariances. See the Wikipedia website
http://en.wikipedia.org/wiki/Mahalanobis_distance
In Mathworks' 'mahal' function
d = mahal(Y,X) ,
that distribution is approximated from the X array, which must have more rows than columns to be meaningful. In your case you were trying to use only one row in the second argument and that would not give a meaningful distribution. I suggest you carefully read the documentation at:
http://www.mathworks.com/help/stats/mahal.html
