from my own understanding, it depends on the period of your OPT. for example:
1st situation: you graduate in May 05, 2006 and your OPT Period is from May 05, 2006 to May 04, 2007. in this case you have two months grace period after your OPT expired.
2nd situation: you still graduate in May 05, 2006, but your OPT period is from July 05, 2006 to July 04, 2007. in this case you don not have any grace period.
hopefully this helps.