  We show that the delay of structure formation from WMAP3 can not fully
account for the reduction of electron optical depth from WMAP1 to WMAP3 when
the radiative transfer effects and feedback mechanisms are took into account in
computing the reionization history of the Universe. As the ultimate limit in
constraining the reionizatin history of the Universe with Planck will be placed
by the our understanding of systematic effects and foregrounds removal, we
discuss also these aspects.
