A detailed account of what is going on can be found here. It is the April 9th press release of ELSTAT (if you happen to be an IB Economics student please also read the methodological notes at the bottom of the page - they are pretty illuminating on how the CPI is constructed).
This was something widely expected as Greece is in a deep-deep-deep recession. Output has been contracting for more than 5 years, official unemployment is over 27%, youth unemployment is over 50%, money wages are cut and it is more of a surprise that deflation is happening now and not a lot earlier.. The answer lies in the very concentrated and still over-regulated product markets where collusive behavior seems to have been the norm. It is in the service sector where prices are now dramatically falling pulling the average price level down (see the ELSTAT press release above). We shouldn't also forget the successive increases in indirect taxes as well as the role of energy prices.
These articles are good for anyone interested in the issue:
Deflation takes hold in Greece (from the FT)
Greece enters deflation for first time in 45 years
(photo is from the Telegraph article)