When typing “outraster = [inlayer]” into the Raster Calculator, the result added to ArcMap is not “outraster”, but a temporary grid with a different name. The permanent “outraster” is saved to disk.
Spatial Analyst creates a temporary raster based on the expression you type. In this case, “outraster = inraster” creates two temporary rasters, outraster and inraster. Because only one output can be added to ArcMap, inraster is added and outraster is saved permanently to disk.
Don't specify a name for the output raster in the Raster Calculator. Spatial Analyst 8.1 allows you to not have to specify an output name when typing expressions in the Raster Calculator. For instance, typing [inlayer] will create a temporary output raster equal to inlayer.
This will be modified in ArcGIS Version 8.1.2, such that typing "outraster = inraster" will result in one permanent grid created (outraster in this case) and added to the table of contents of ArcMap.