**This issue has been addressed in ArcGIS 8.1.2.**
When appending a raster to an existing raster in a geodatabase, the following error may occur:
"Image mosaicking is not allowed."
The two rasters may not be aligned cell-wise, have different cell sizes, be of different data types (different bit depth), or the existing raster in the geodatabase does not have a spatial reference specified.
In ArcGIS 8.1, use datasets that align perfectly (don't underlap or overlap), have the same cell size, have the same data type, and have the same coordinate system.
Some sample scripts to workaround this limitation can be found on the ArcObjects Online Web site.