Hi Tony,
The Shortest Path tool and the Cost Path GP tool are the same tool with different names. I don't have 9.x installed to take the screenshots necessary to highlight this so I'll just point you to some documentation.
Under Analysis Concepts > Distance Analysis > Least Cost Path and Least Cost Corridor you'll find generic information regarding what cost path/corridor analysis is and how to perform it in ArcGIS. On this page you will find instructions (Links) to Learn how to calculate least cost path using the Spatial Analyst toolbar and to Learn how to calculate least cost path using the Cost Path tool
In the Usage Tips section (for the GP tool documentation) it states the following:
"Cost Path can also be used to derive the path of least resistance down a digital elevation model (DEM). In this case, the DEM will be the input cost distance raster and the flow direction raster will be the input cost backlink raster. Valid flow direction raster values are 1, 2, 4, 8, 16, 32, 64, and 128; valid values in the backlink raster are 1, 2, 3, 4, 5, 6, 7, and 8. Both of these rasters are acceptable."
Attached are screenshots of a simple model I made and the results overlaid on a hillshade to give you some perspective.