When calling IRasterGeometryProc::Shift with positive x and y values as the parameters, the result is shifted in the opposite direction (as if the x and y were negative).
This is a known problem with ArcGIS 8.1. The internal sign used to determine the shift direction was incorrect.
In ArcGIS 8.1, use the opposite sign for the parameters. For example, if you want to shift to the positive x direction, use parameter -x.