There are multiple operating systems used in mobile devices, thus making it very difficult to support all. Current OSs include:
Open Source:
     Google Android (Open Handset Alliance)
     LiMo Foundation (Linux)
     Nokia Symbian
Vendor:
     Microsoft Windows Mobile and to come Windows Phone 7
     RIM
     Apple
     Palm
Also potentially important to development... What about Adobe Air, Flash, Java or Silverlight? Think about moving to HTML5.

Smart phone subscribers by platform in the US as of May 2010 according to ComScore were:
RIM - 41.7%
Apple - 24.4%
Microsoft - 13.2%
Google - 13.0%
Palm - 4.8%

Subscribers in the US by device as of May 2010 according to ComScore were:
Samsung - 22.4%
LG - 21.5%
Motorola - 21.2%
RIM - 8.7%
Nokia - 8.1%