  We discuss new high-field magnetization data recently obtained by Tsirlin et
al. for layered vanadium phosphates in the framework of the square-lattice
model. Our predictions for the saturation fields compare exceptionally well to
the experimental findings, and the strong bending of the curves below
saturation agrees very well with the experimental field dependence. Furthermore
we discuss the remarkably good agreement of the frustrated Heisenberg model on
the square lattice in spite of the fact that the compounds described with this
model actually have a lower crystallographic symmetry. We present results from
our calculations on the thermodynamics of the model on the orthorhombic (i.e.,
rectangular) lattice, in particular the temperature dependence of the magnetic
susceptibility. This analysis also sheds light on the discussion of magnetic
frustration and anisotropy of a class of iron pnictide parent compounds, where
several alternative suggestions for the magnetic exchange models were proposed.
