The CO-H2 conversion factor of diffuse ISM: Bright 12CO emission also traces diffuse gas

5 pages, 1 PostScript figure. To be published in the proceedings of the Zermatt 2010 conference: ``Conditions and impact of star formation: New results with Herschel and beyond''. Uses EAS LaTeX macros.We show that the XCO factor, which converts the CO luminosity into the column density of molecular hydrogen has similar values for dense, fully molecular gas and for diffuse, partially molecular gas. We discuss the reasons of this coincidence and the consequences for the understanding of the interstellar medium