I'm absolutely a Nokia fan when it comes to cell phones but still i would not waste time programming for Symbian although C++ is native to it.
My point is that Microsoft is the shizzle when it comes to 'programmable' systems... by that I mean most of the software market targets Microsoft (applications, games, entertainemnt...etc). And so by doing a very 'programmable' hand-held computers (something similar to iPhone), they would boost the world of software market that targets their platforms plus more importantly, they would attract C++ and C# programmers to do their stuff instead of doing Objective-C for the apple.