Symbian is way down because they have not had a competitive high end smartphone product for at least a year and are in transition from S60 to Symbian so product releases have been quite stagnant. RIM sales look to have been fairly steady, but companies that issue BlackBerry devices do not move as fast as consumers and some people have been waiting for BB 6. After using a BB Torch for a few days I think RIM will stay pretty flat moving forward and most likely will continue losing market share. Apple's iOS slowed down significantly, but with their once a year release this was expected. I am sure the next quarter numbers will show a rise with the successful sales of the iPhone 4. Windows Mobile has continued to lose significant market share with no OS update as we all wait for WP7.
Android is successful because they provide a compelling OS that is available on every carrier in several different form factors. There is a lot of choice for the consumer and prices have fallen to a standard of $200 for high end subsidized smartphones. Android launched on the T-Mobile G1 in late 2008 and was pretty flat for at least the first year with few devices and few carriers with Android devices. The Verizon Motorola Droid really put Android on the map and generated the kind of excitement I expected with such a powerful OS. Google has continued to update and improve the OS too.
I think Microsoft has the opportunity to do the same thing as Android with their multi-carrier support, various form factor choices, and a compelling operating system that is focused on the end user experience while still meeting the needs of enterprise customers. The latest BlackBerry 6 OS is still not very touch friendly and I think Windows Phone 7 will be more appealing to those looking for a fun device to also use for getting work done on the go. Like Android, sales may be a bit flat at first while people come back to a Microsoft product, but Microsoft is in it for the long haul and as more devices and updates appear I think sales will increase significantly.