The thing is, they are losing their grip on the business market. In terms of business mobile systems, Blackberry have most of that, and the network server market has always been pretty diverse, thanks to Linux.
When it comes to business desktop workstations, it's honestly too soon to tell yet, since there's so much lag between a new product coming out and customers buying it. Businesses don't upgrade to a new OS until it's been out for at least a couple of years, so it's way to early for many to have got Windows 8 yet. But in the light of MS' recent problems, and the increasing security risk of using W8, it does seem highly likely that Windows is going to lose market share there too.
Incidentally, does anybody else think that the Ubuntu mobile OS could be a real hit with business users if they incorporate the necessary security and network controls? Integrating the mobile and desktop computers into one device could be really popular.