I did a simple test and just wanted to share the findings.
When you create a 32 bit application and run it on a 64 bit machine (i.e. you run the application under WOW64). If you create a process from inside it - the process that you create is a 32 bit process. If you compile the same application as 64 bit (either x64 or IA64) and do a create process for the same process - the child process will be created as 64 bit.
One more side note: if you create a 64 app in Visual Studio on a 64 bit machine -- you cannot run/debug the application from inside the IDE. This is because the Visual Studio is a 32 bit process (we do not have a 64 bit version of VS yet). You wil have to launch the application from the Windows Expolrer/Command Prompt. To debug it you may want to use the 64 bit version of WinDBG.