How likely will it happen on that day is difficult to say because the European and the US will do everything in their power to postphone the day of reckoning, because a greek default has the potential to bring down many large European and US banks.
Here is a link to an article in ETF daily.
http://etfdailynews.com/2012/02/16/greece-officially-defaults-march-23-banks-close-ewg-vgk-fxe-ewq-gld/