Symbian and WinMo both falling, Winmo will hit the dirt first
Symbian and Windows Mobile are the two phone platforms that have a falling market share. Both were designed for use with stylus pens, rather than multi-touch. Both need to re-engineer their interfaces for multi-touch (HTC placing a cover over Windows Mobile in an attempt to hide WinMo as much as possible doesn't count).
With 45-50% of the mobile market, Symbian has some falling space. Its huge market share provides some padding, and allows it time to sort itself out, and improve the interface to be more touchy-feely.
Windows Mobile, on the other hand, is in a death spiral from which it cannot recover. WinMo's market share is falling very fast fast, but with just 8.8% market share (current Canalys estimate), WinMo is about to splatter on the tarmac. Most analysts, including Gartner, expect Microsoft to withdraw from the mobile market in 2010 or 2011. Windows Mobile is dead, so it's no wonder Microsoft can't find many developers to write applications for its deserted Mobile Marketplace.
Although Windows Mobile is now finished, with no hope of revival, Symbian will soon have another competitor eating its lunch. That's the Linux-based Maemo phone OS. Both OSes have the same mother (Nokia), and Maemo is the preferred child. Being Linux-based, Maemo will probably receive more love from the open-source community.