  AIMS. Several authors have claimed to detect a significant cross-correlation
between microwave WMAP anisotropies and the SDSS galaxy distribution. We repeat
these analyses to determine the different cross-correlation uncertainties
caused by re-sampling errors and field-to-field fluctuations. The first type of
error concerns overlapping sky regions, while the second type concerns
non-overlapping sky regions. METHODS. To measure the re-sampling errors, we use
bootstrap and jack-knife techniques. For the field-to-field fluctuations, we
use three methods: 1) evaluation of the dispersion in the cross-correlation
when correlating separated regions of WMAP with the original region of SDSS; 2)
use of mock Monte Carlo WMAP maps; 3) a new method (developed in this article),
which measures the error as a function of the integral of the product of the
self-correlations for each map. RESULTS. The average cross-correlation for b>30
deg. is significantly stronger than the re-sampling errors--both the jack-knife
and bootstrap techniques provide similar results--but it is of the order of the
field-to-field fluctuations. This is confirmed by the cross-correlation between
anisotropies and galaxies in more than the half of the sample being null within
re-sampling errors. CONCLUSIONS. Re-sampling methods underestimate the errors.
Field-to-field fluctuations dominate the detected signals. The ratio of signal
to re-sampling errors is larger than unity in a way that strongly depends on
the selected sky region. We therefore conclude that there is no evidence yet of
a significant detection of the integrated Sachs-Wolfe (ISW) effect. Hence, the
value of Omega_\Lambda ~ 0.8 obtained by the authors who assumed they were
observing the ISW effect would appear to have originated from noise analysis
