[DllImport("iPAQUtil.dll", EntryPoint = "iPAQGetSerialNumber")]
private static extern bool iPAQGetSerialNumber([Out] StringBuilder SerialNumber);

Notes:

Tips & Tricks:

No tips or tricks required, should work first time!

Sample Code:

public static string GetIPAQDeviceID()

{

    StringBuilder sSerial = new StringBuilder(40);
    if (iPAQGetSerialNumber(sSerial) == false)
    {
        return null;
    }
    else
    {
        return sSerial.ToString();
    }

}