I think that the question "Why" has been a little bit forgotten during recent events and I do suspect that there's a very simple reason for Nokia's current actions:

As everyone probably is aware about, Nokia has had difficulties in developing Symbian quickly enough to stay in touch with the competitors.

Since ARM is working on multicore CPUs and Symbian's kernel is designed for traditional single core CPUs, Nokia faced the need to rewrite Symbian's kernel to be able to utilize these new CPUs. The folks at Nokia have apprently long ago realized that Nokia doesn't have the resources to rewrite the Symbian kernel (at least fast enough) thus leaving Nokia with an operating system which can't utilize these new multicore CPUs. The anouncement to ditch Symbian^4 and upgrade current Symbian^3 in increments was probably a consequence of this realization.

My conclusion is that Micro$oft was the least bad alternative of only bad ones. Time will tell. But I wonder what will become of Meego on Nokia's horizon?

Personally I think I'll buy another N900 and mothball it so that I'll have a working überphone when my current one is worn out. It's very possible that the N900 will be the entire phonemarket's first, only and last überphone .....