Because whatever gets approved in April-Jun-Jul ... - October 2007 is federal fiscal year 2008 which starts from Oct 1 2007. You can work if you are currently authorized (CPT,OPT ect) to work which is valid through Oct 1st. otherwise, its not only that you cannot work, either you maintain your status by 1. extending it (if allowed), 2. changing to other status than currnet 3. or by leaving the courntry temporarily. H1B approved from April-October 2007 is valid only from 1st october 2007 (both for status and to apply visa to enter the US)