Microsoft have reversed their policy/recomendation on the use of the /NOPAE switch in the boot.ini file of servers running Windows 2003 with Exchange 2003.
Microsoft's previous policy/recomendation was to set the /NOPAE switch in the boot.ini of servers running Windows 2003 with Exchange 2003. The Exchange Server Team have now found no issues running the PAE kernel or running with DEP enabled on these setups.

To read more about this issue, have a look at this article on the Exchange Server Team Blog.