  The main uncertainty in current determinations of the power spectrum
normalization, sigma_8, from abundances of X-ray luminous galaxy clusters
arises from the calibration of the mass-temperature relation. We use our weak
lensing mass determinations of 30 clusters from the hitherto largest sample of
clusters with lensing masses, combined with X-ray temperature data from the
literature, to calibrate the normalization of this relation at a temperature of
8 keV, M_{500c,8 keV}=(8.7 +/- 1.6) h^{-1} 10^{14} M_sun. This normalization is
consistent with previous lensing-based results based on smaller cluster
samples, and with some predictions from numerical simulations, but higher than
most normalizations based on X-ray derived cluster masses. Assuming the
theoretically expected slope alpha=3/2 of the mass-temperature relation, we
derive sigma_8 = 0.88 +/-0.09 for a spatially-flat LambdaCDM universe with
Omega_m = 0.3. The main systematic errors on the lensing masses result from
extrapolating the cluster masses beyond the field-of-view used for the
gravitational lensing measurements, and from the separation of
cluster/background galaxies, contributing each with a scatter of 20%. Taking
this into account, there is still significant intrinsic scatter in the
mass-temperature relation indicating that this relation may not be very tight,
at least at the high mass end. Furthermore, we find that dynamically relaxed
clusters are 75 +/-40% hotter than non-relaxed clusters.
