Abstract
X-ray cluster measurements interpreted with a universal baryon/gas mass fraction can theoretically serve as a cosmological distance probe. We examine issues of cosmological sensitivity for current (e.g. Chandra X-ray Observatory, XMM-Newton) and next-generation (e.g. Con-X, XEUS) observations, along with systematic uncertainties and biases. To give competitive next-generation constraints on dark energy, we find that systematics will need to be controlled to better than 1% and any evolution in fgas (and other cluster gas properties) must be calibrated so the residual uncertainty is weaker than (1+z)0.03.