  The universal baryonic mass fraction (Omega_b/Omega_m) can be sensitively
constrained using X-ray observations of galaxy clusters. In this paper, we
compare the baryonic mass fraction inferred from measurements of the cosmic
microwave background with the gas mass fractions (f_gas) of a large sample of
clusters taken from the recent literature. In systems cooler than 4 keV, f_gas
declines as the system temperature decreases. However, in higher temperature
systems, f_gas(r500) converges to approx. (0.12 +/- 0.02)(h/0.72)^{-1.5}, where
the uncertainty reflects the systematic variations between clusters at r500.
This is significantly lower than the maximum-likelihood value of the baryon
fraction from the recently released WMAP 3-year results. We investigate
possible reasons for this discrepancy, including the effects of radiative
cooling and non-gravitational heating, and conclude that the most likely
solution is that Omega_m is higher than the best-fit WMAP value (we find
Omega_m = 0.36^{+0.11}_{-0.08}), but consistent at the 2-sigma level.
Degeneracies within the WMAP data require that sigma_8 must also be greater
than the maximum likelihood value for consistency between the data sets.
