Rarely, if ever. Most agreements clearly state that miles are not given for convenience check purchases, the exception being introductory offer balance transfers.

There are threads in the credit card forums about paying for cars with cards. Technically, according to the Visa / MC agreements, if they accept a card for partial payment, they must also accept the card for full payment as well. That's how I understood it.

Best thing to do is this, barring any troubles: Pay for the entire car with credit card A. Then transfer that balance to credit card B with a 0% balance transfer / $0 fee promo. Or even a $75-$150 fee. Still good way to get thousands of miles. Though this is probably much harder to do now than it was a year ago, considering such card deals are almost non-existent for many now.