So far as I know if you are filing for graduation you can apply for OPT and obtain it. One of my friends is defending on July mid and she got her OPT effective from June and there is another girl she is already on her OPT but she is defending her Thesis in Fall 2008. I think filing for graduation is important but this depends on your thesis supervisor/graduate coordinator  who will  be  willing  to approve your  graduation  filing/application.  And  this will depend  on your  results  with research data, which stage of thesis you are in .......