Hi,
In 9.3.1, the Raster calulator in the Spatial Analyst Toolbar could be used to merge rasters. The syntax for merge in raster calculator is: merge[[Raster1],[Raster2],...]. Click Evaluate and a temporary file is automatically added into ArcMap. Right click on the temporary file and select Data > Make Permanent.
In 10.0, the merge function is not available in the raster Calculator. However, the Mosaic tool and Mosaic To New raster tool (Data Management Tools > raster > raster dataset) can be used to merge rasters (you can also use these tools to merge rasters in 9.3.1).
Regards,
Gayu