You should be able to save/export from 123D to your computer as an STL then upload to Shapeways to 3D print.
Duann Scott, Designer Evangelist, Shapeways
Hi Ayewondery and duann,
we added "My Corner" storage to 123D Beta 9. "My Corner" allows you to automatically save your designs in the cloud, for usage with other 123D applications.
Once you are logged-in (upper right hand corner), you will be able to export to STL locally.