Orthorectification is a process that "georeferences" all the pixels in an image as well as corrects for distortion caused by the camera and terrain. 9.1 (arcgis ???) does not have the tools to perform true orthorectification of images. The Georeferencing tools in 9.1 are a suitable method to georeference images (raster data), but remeber that the process is not true orthorectification and there is still distortion present even after an image has been georeferenced.
Graham Matthews & Associates