Per PayPal's site itself on the cashback:
Financial Products - PayPal Debit Card
When did the PayPal Preferred cashback change from 1.5% to 1.0%?
On March 24, 2005, the cashback bonus for PayPal Preferred members changed from 1.5% to 1.0%. All members with the PayPal Preferred status on or before March 24, 2005, will continue to receive 1.5% cashback as long as they continue to meet the PayPal Preferred Requirements, and their accounts remain in good standing.