Amazingly, Microsoft's server market share continues to grow, according to IDC. CNET reports that the growth in Linux server sales does represent a significant challenge to Microsoft's servers over the long term, but because other proprietary vendors, like Sun, are stumbling, Microsoft's dominance is increasing for the time being.
Even Windows desktop OSes grew in total share, reaching 93.8 percent of desktop OSes shipped. I'd like to know what proportion of those Windows shipments were replaced by Linux once the customer received them -- it would be an interesting statistic to know.