It'll be nokia. Yes symbian was failing so they agreed to use windows 7 because Microsoft and nokia struck a deal which would benefit Microsoft. Ms is having problems getting people to switch to their mobile OS so they wanted to sign a phone maker. Nokia was up for it because it would help them dig themselves out of a hole.... theoretically.
Thing is, wp7 isn't going to sell like microsoft wants it to - anybody here remember when they tried to take on the iPod with the zune? Same thing is happening - a ton of advertising about simplicity and such but no results.
Once nokia wises up, you'll see the little green robot in their phones.
Blackberry won't switch anytime soon but they will cooperate with android because it'll help their sales and appeal.
Windows won't switch on premise: google and windows compete with everything - browsers, search, pc OS, mobile OS, etc.