More articles on the story which was initially reported in May of 2005 -
- same article reported in ScienceDaily.
I first saw news about on a website concerning non-destructive testing - in this case with X-rays (radiography).
Archimedes Manuscript - The Palimpsest
AFAIK, one uses 'soft' X-rays. The technique can also be used to reveal paintings which have been overpainted because some artist ran out of canvas. And it can be used to detect fraudulent paintings and ancient manuscripts.