This was a bug. It's fixed in ArcGIS 10.1.
You can convert the rst file into tif or img file, then open it in ArcMap 10.
To convert, go to File->Import->GDALIDRISI, in tab gdal_translate, input is the rst file, provide output, and for output format, choose GTiff or HFA. After done, you can open the output file in ArcMap 10.