If you are familiar with Vistaprint then you probably know users can upload a .eps, or .ai file (along with other types) and see a proof on screen and/or as a PDF.

I am trying to find information on how to do this.  I would like a web application that can read vector based files as listed above, show a proof, and if possible write back to the vector based file.

I cannot find any information though.  Any help would be appreciated.