Some say it makes no sense, except if Nokia was about to switch to Android or to bankrupt.
Cringely says it's all about money-laundering.
A long summary from Tomi Ahonen.
Some are visioning the end of Windows as standalone product.
And now everybody is like Apple. Except that they are not (see below).
I, personally, think Microsoft just made another non-decision decision. Now their mobile ecosystem is neither Apple-like nor Android-like. It's the mix of the two and that is simply wrong.
And by the way: the step down of Steve Ballmer is the greatest news of 2013, or maybe even the next decade. Vista, Longhorn, mobile, Nokia, tablet, Silverlight, and just recently he tried to ruin the Xbox business. Whatever comes, the future can only be better.
UPDATE: I've found a good article describing Microsoft's mixed "strategy".