USCIS has confirmed that if they determine that they have received enough number of cases in the first five business days of April to reach the cap, then the "lottery" will be based on petitions received on all five days. Note that the first five business days include upto 7th April.
USCIS has said that they will not begin to issue fees receipts, until sufficient H-1B petitions have been received. After the lottery is conducted, USCIS will then issue receipts for those cases that are selected, and the receipts will probably all have the same receipt date: April 8, 2009. If, however, the quota is not filled by April 7, they will continue to accept H-1 filings.