>>You will be limited by the CLR versions available for the different smartphone OS's. In practice this means that have a limited range of available functions since the CLR versions for smartphones other than those running Windows, are relatively old.
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>Considering the .NET line for development, what would you recommend for development apps for smartphones?

I'm sorry, but I can't answer you. My understanding is that you will most likely end up with different versions of your application for the different smartphone OS's.