They still have a lot of influence in business computing - from PCs to servers. Don't be so quick to dismiss Microsoft.
It doesn't matter if the tablets are running iOS, Android, or Windows. What does and will matter is, which can run Excel, which can run Word, and which can run Eclipse or Visual Studio? Because, besides IE6 apps that will continue to slowly fade, that's what businesses actually care about.
That's an open question. I would ask instead which ecosystem - Google, Bing/Hotmail/SkyDrive/Azure, Apple, Yahoo, etc. the customer uses as their "home" ecosystem.