Yes. Your new ArcGIS 8.1 license file along with an ArcGIS 8.1 license server will manage earlier ESRI software. ArcInfo 8.x, ArcInfo 7.2.1, ArcView GIS 3.1 and higher, and SDE 3.0.1 and 3.0.2.x are supported, provided the license information is correct for each ESRI product you wish to use.
You will need to install the license manager that comes with ArcGIS 8.1 to obtain the 8.1 tools, documentation and most recent version of the license manager.