PIA has released the study on digital printing and survivability in the U.S. Postal:
How well does a digitally-produced postcard hold up in the United States Postal System mail stream?
The basic methodology for the study: design a postcard, print it on various digital presses (with no coating) on 10pt C1S paper, then mail it from four different points of origin to Printing Industries of America headquarters. Additionally, a postcard was also produced via offset lithography for control and comparison purposes. Results show that both the press used, as well as the postal center through which the piece was processed, significantly affected the amount of scuffing. The full white paper details the complete results of this study.
Download the study from PIA: Digital Printing and Survivability in the U.S. Postal System (PDF link).