I didn't think the new United card offered double miles on anything other than United flights. They certainly don't advertise it as such. But I just got an email about my Mileage Explorer card that contains the following fine print:

TERMS AND CONDITIONS:
1 You will earn 1 mile for each $1 of net purchases. You will earn 1 additional mile for each $1 of net purchases made directly from United and on tickets purchased directly from Continental and at continental.com. You will also earn 1 additional mile for each $1 of net purchases made at merchants that classify their merchant locations for Visa/MasterCard in any of the following categories: office supply stores and wholesale distributors, dining establishments, gas stations. Purchases not eligible to receive the 1 additional mile on category purchases include, but are not limited to, purchases made at superstores, warehouse clubs, discount stores, convenience stores, and their affiliates. Online and catalog purchases are not eligible to receive the 1 additional mile on category purchases unless the merchants have correctly identified and billed the transactions. We do not determine whether merchants correctly identify and bill transactions as being made in each category. However, we do reserve the right to determine which purchases qualify for the 1 additional mile on category purchases.