Autodesk has just updated the online version of 123D Design to add even more features that make it perfect for design for 3D printing with Shapeways.
The latest iteration includes tools so that you can fillet and bevel edges, shell a part to help define wall thickness that is critical to ensure your design is 3D printable in the material of your choice. 123D Design also has a design history (not a tree) that means you can go back to a geometry you have created and edit it in a parametric way at any point in the process. As well as solid modeling from scratch you can also import existing 3D meshes in STL and OBJ format which you can then edit and tweak to then prepare to 3D print. If you have a 2D vector drawing that you want to bring into the 3rd dimension you can also import SVG files into 123D Design to extrude and edit.
Take a look at this quick video below to see an example of the interface and some of the tools to design for 3D print.