I think that if during a time period there are no trades then the open,close,high,low is equal to the previous period close price.
This chart was during the time mtgox was down for a few days, and its just as I described.
http://bitcoincharts.com/charts/mtgoxUSD#rg360zigDailyzczsg2011-06-10zeg2011-07-05ztgSzm1g10zm2g25zv