Windows Phone is implenting homebrew compatibility after the unlock program came out. Don't know how it's going to play out but it's simple design is liked by a lot of people who use smartphones for business/social and all those purposes.
Can't say much about android except it's eyecandy...oh and the kids love it, teen market.
Symbian has developed very well but still behind any market.
Windows mobile is dead period, I still use it and customize it to no end but there is no support yet MS still releases phones installed with WinMO.
iOS has taken over the world. Sleek black look rules all and high quality apps. Yet so many restrictions and bad hardware.

Don't know to be honest they are all having trouble getting to the zephyr, 07-10 iPhone ruled all.

Now it could be Windows Phone+Manila 3?? Since HTC is releasing HD7 the market might have a whole lot of interest in WP7.

Let's look to see if nokia phone is able to septuple boot all these in the near future.