In October 2010, we have started a free online service - STL2DWG - for web-based conversion of 3D stereolithography files (.STL) back into CAD applications. It would be sexy to call this a "web-cloud service" but it currently runs on our own physical CAD Forum internet server, so there are really no scaleability, elasticity or other cloud-related attributes involved. Despite this boring "no-cloud" nature, this online tool got rather popular among CAD users and has successfully converted already more than 3.000 STL models uploaded by CAD Forum users.
Neither AutoCAD, nor Inventor and Revit can import STL files natively. These CAD applications can only export STL files as a part of their "3D printing" functionality. So when you need to use (re-import, re-engineer) an existing .STL file for viewing, measuring or reconstructing of 3D geometry, you can use this online service available on the CADforum.cz portal.
How does it work? Just upload your textual or binary STL file from any source and this web tool generates and returns a SCR (script) file for AutoCAD (to save it to DWG), or a DXF file compatible with virtually any 3D CAD application.
To load the resulting DXF file into AutoCAD, you can just use Open after you click on the result of the online conversion. To load the 3D DXF file into Inventor, download the file and use the Inventor import wizard (Open) and check its options "3D solids", "Use default file names" and "New part, Wires to 3D sketch". In Revit, you can load the DXF file with the ribbon function Insert > Import > Import CAD.
You can try this online tool at STL2DWG