Jeff,
http://matplotlib.svn.sourceforge.net/viewvc/matplotlib/trunk/toolkits/basemap/lib/mpl_toolkits/basemap/init.py?r1=5520&r2=5534
Lines 2311 and 2312 in changeset 5520 have bitten me badly, and I think they are ill-advised. They are based on the assumption that input to rotate_vector, which must be called before m_quiver, should be describing a rectangular array of points; but this is not the case for a typical oceanographic use-case, plotting current vectors as measured along a ship track. In that case one naturally has 4 1-D vectors of the same shape for U, V, X, and Y, and one does not want meshgrid to be called on X and Y. If you want automatic meshgrid, it should be called only if X and Y are 1-D *and* their lengths match the 2-D shape of U and V.
Eric