I just got the spam about Autodesk 123D -Beta5 being out.
They list IGES export and STL import+export.
I try not to get my hopes that high but it just may work as an STL to IGES converter.
Will have to try if that works ASAP!
Result: 123D can work with meshes and solids in the same project but not export a mesh as a solid.
This could be nice to build assemblies of solids with part that I only have as meshes.