There are couple of Pinvoke metthods used for opening the custom keypad window without a focus on it. This API works in CF1.0 however it doesn't work for .NET CF3.5 Following is the correct one for .NET CF3.5
[DllImport("coredll.dll", SetLastError = true)]
private static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);
[DllImport("coredll.dll", SetLastError = true)]
private static extern int GetWindowLong(IntPtr hWnd, int nIndex);