None that I can think of - as long as either the 2007 versions are set to
use 97-2003 documents by default, or, the 2003 versions have the
Compatibility Pack installed...
If you mean on the same PC;
Only a single instance of outlook can be installed
2007 will overwite 2003 shortcuts, unless you first modify 2003 shortcuts
prior to the install of 2007
Generally its not neccessary since 2007 can create docs in the 2003 format