If your cell phone runs on Windows Mobile you can program for it using Visual Studio using any CLR supported language such as C#, VB.NET or J# which cuts down on the development time considerably. For phones like iPhone you have to code in C or Java. Same for the G1 phone that runs on the Google Android OS.
Further reading:
Windows Mobile:
Windows Mobile Developer Center
iPhone:
iPhone Dev Center - Apple Developer Connection
G1:
The Developer's Guide | Android Developers
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