The following has just been published in the journal Nature Communications and can be read on-line:
Vito Mocella, Emmanuel Brun, Claudio Ferrero, and Daniel Delattre, Revealing letters in rolled Herculaneum papyri by X-ray phase-contrast imaging. Nature Communications 6, article no. 5895 (publ. 20th January 2015).
There is also an article on the BBC website about this.