Every time I have used 1-800-CHECKIN, they have required the credit card upon arrival. You are not "checked-in" until then.

I recently executed a phanton check-in by prepaying the Hyatt.com rate. I used 1-800-CHECKIN as well. When my travel plans changed, I called the hotel to explain on the night of check-in. Somebody at the front desk agreed to check me out and FAX a copy of the bill for backup.

This worked for FFN because it was a Hyatt.com prepay on Mastercard. I understand that all hotels may not cooperate in this manner.