Unfortunately, ArcCatalog does not respect the Editor and Debugger Geoprocessing Options when you edit a ToolValidator.Make a registry change (Windows 7) to change the default editor (for me it was IDLE) for the ToolValidator class.
Add an edit key to the shell key for Python.File:
Create a new string value, and in the value put the path for your editor, for example if you set Notepad++:
C:\Program Files (x86)\Notepad++\notepad++.exe "%1"
Geoprocessing Options are ignored when editing the ToolValidator class: