  We have studied the implications of high sensitivity polarization
measurements of objects from the WMAP point source catalogue made using the VLA
at 8.4, 22 and 43 GHz. The fractional polarization of sources is almost
independent of frequency with a median of ~2 per cent and an average, for
detected sources, of ~3.5 per cent. These values are also independent of the
total intensity over the narrow range of intensity we sample. Using a
contemporaneous sample of 105 sources detected at all 3 VLA frequencies, we
have investigated the spectral behaviour as a function of frequency by means of
a 2-colour diagram. Most sources have power-law spectra in total intensity, as
expected. On the other hand they appear to be almost randomly distributed in
the polarized intensity 2-colour diagram. This is compatible with the polarized
spectra being much less smooth than those in intensity and we speculate on the
physical origins of this. We have performed an analysis of the correlations
between the fractional polarization and spectral indices including computation
of the principal components. We find that there is little correlation between
the fractional polarization and the intensity spectral indices. This is also
the case when we include polarization measurements at 1.4 GHz from the NVSS. In
addition we compute 45 rotation measures from polarization position angles
which are compatible with a \lambda^2 law. We use our results to predict the
level of point source confusion noise that contaminates CMB polarization
measurements aimed at detecting primordial gravitational waves from inflation.
We conclude that some level of source subtraction will be necessary to detect
r~0.1 below 100 GHz and at all frequencies to detect r~0.01. We present
estimates of the level of contamination expected and the number of sources
which need to be subtracted as a function of the imposed cut flux density and
frequency.
