You keep mentioning Smartphone here, and the Smartphone SDK. The XDA does not support the Smartphone SDK, as it is not a Smartphone - it runs Pocket PC 2002 Phone Edition - something completely different.
So please, don't spend several hours downloading the Smartphone SDK to find it's not the right one. Download the Pocket PC 2002 SDK. I have developed several apps for the XDA using this already.
I'd like to throw in another suggestion: the .Net Compact Framework. If you're a Delphi programmer (as are we - I used to be on TeamB for Delphi), you'll take to it straight away. After all, .Net and C# was designed by the same Anders Hejlsberg that designed Delphi. C# is very like Object Pascal with a C/Java syntax, but with even more goodies.
We've been using the Compact Framework beta for several months and it is quite simply superb. It was just launched officially on April 26th as part of Visual Studio.Net 2003. However, you don't need to buy Visual Studio - just download the .Net 1.1 SDK from Microsoft - it's free.
It's just a subset of the full .Net Framework, but if you need to do something that's not supported directly in the Framework classes, you can easily call API functions - or even write some code in embedded VC++ and call that. The managed environment is just great.