The most complete description of this offer is located in this thread at FW. Note, however, that this deal is no longer available online. You will have to call in with the invitation codes specified in this thread at FT. In short:

• Apply for the Platinum Business Premium Cash Rebate Card at 866-764-2177, using one of the following RSVP codes (Note: It’s unclear whether these codes still work):
• 7EUZC-BCB
• 7UECI-BCF
• Receive $250 statement credit after your first purchase
• If, 6-8 weeks after your first purchase, you haven’t received your $250 statement credit, call AMEX Open Customer Service mentioning the following “Promotional Code Offer ID” you applied for:
• Promotional Code Offer ID: E341-0001
• Phone #: 1-800-492-3344
• You may need to mention the RSVP # as well.

Due to the high levels of interest in this promotion, AMEX may attempt to validate that you indeed have a small business.

If you are eligible and can get this promotion, it is definitely one of the best credit card signup deals of all time.