As per Microsoft support, http://support.microsoft.com/kb/894435, running your 32 bit ASP.NET applications in IIS on Windows Server 2003 x64 is a simple matter of flipping the bit and enabling 32 bit mode. After aspnet_regiis on framework, you should be ready to go.
Having said that, has anyone come across post production/deployment issues in said configuration?
I know, I know...what's the justification for running such a scenario? You could have an internal library that references 32 bit specific dll (native code) in which you don't have the 64 bit version of. Which leads to the next thought..why not use Windows Server x86 with PAE extensions? What are the benefits of Server with PAE as opposed to Server x64? Comments, thoughts and rants are welcome. Answers ranging from "Depending on if x and y are...you will get..." vs. general practices.