With the first Nokia/WP phones slated for 2012, there is ample time for one (if not two) updates for iOS phones and a boat load of Android (especially low cost) devices to hit the market. With no meaningful transition (for both customers and developers) from Symbian to WP, why would anybody buy a high end Symbian device today ?
There has been a lot of chatter about a cheaper iPhone being able to penetrate emerging markets. I suspect unless that device can work without requiring a computer, this will be a non-starter. Android devices have the edge in this regard.