Once auditing is turned on, you can view the built-in reports here: /_layouts/Reporting.aspx?Category=Auditing
It looks like running the custom reporting allows you to view the following things:
- Opening or downloading documents, viewing items in lists, or viewing item properties
- Editing items
- Checking out or checking in items
- Moving or copying items to another location in the site
- Deleting or restoring items
- Editing content types and columns
- Searching site content
- Editing users and permissions
- Editing auditing settings and deleting audit log events
- Workflow events
- Custom events
I don't think "Editing the Navigation" would follow under any of those things.
Would this have to something custom written using SPAudit? Are there any examples out there of auditing the navigation changes in SharePoint (MOSS 2007)?