  We consider the effect of radiation pressure from ionizing photons on black
hole (BH) mass estimates based on the application of the virial theorem to
broad emission lines in AGN spectra. BH masses based only on the virial product
V^2R and neglecting the effect of radiation pressure can be severely
underestimated especially in objects close to the Eddington limit. We provide
an empirical calibration of the correction for radiation pressure and we show
that it is consistent with a simple physical model in which BLR clouds are
optically thick to ionizing radiation and have average column densities of
NH~10^23 cm^-2. This value is remarkably similar to what is required in
standard BLR photoionization models to explain observed spectra. With the
inclusion of radiation pressure the discrepancy between virial BH masses based
on single epoch spectra and on reverberation mapping data drops from 0.4 to 0.2
dex rms. The use of single epoch observations as surrogates of reverberation
mapping campaigns can thus provide more accurate BH masses than previously
thought. Finally, we show that Narrow Line Seyfert 1 (NLS1) galaxies have
apparently low BH masses because they are radiating close to their Eddington
limit. After the radiation pressure correction, NLS1 galaxies have BH masses
similar to other broad line AGNs and follow the same MBH-sigma/L relations as
other active and normal galaxies. Radiation forces arising from ionizing photon
momentum deposition constitute an important physical effect which must be taken
into account when computing virial BH masses.
