They are not the unstoppable force that they were before but I think that calling them irrelevant is a bit too much.
Or look at it this way: It's not about selling a phone OS. It's about the ecosystem behind the devices.
If you don't think Microsoft's ecosystem is competitive, it tells me you haven't tried it.
You probably will never come across them, but one of my cousins got a Lumia, and made the mistake of not lying when registering the phone.
She ran afoul of two things:
1) Marketplace simply does not work if you're a minor
2) Almost nothing works if you set your place of residence to Uruguay.
So I had to create a false account for her, and reset her phone twice, before she had a passable user experience.
It might "work" if you live in the U.S., but I absolutely do not recommend it for small third world countries, and will only recommend Android here.
Btw, I'd add a third caveat:
3) Most software doesn't have "free" alternatives like on Android (absolutely deal-breaking if you don't have access to a credit card, which most Uruguayans don't)
It's sad, because the hardware beats the crap out of Samsung phones, the UX is better, and I love the Camera button. Too bad Nokia chose Windows, I loved my Nokia phones (still use one, an N86).
Apple and Google are the major players now. Microsoft is an also-ran.
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.
Where money is being spent, Google and Apple are no where to be seen. They're just fighting over the fickle and low margin consumer market.
That theory would be more convincing were it not for the fact that Apple makes more money than Microsoft...