This has been discussed in some other forums here. My reading (and personal experience) is that it's pretty hard to get most dealers to take more than the initial deposit/down payment on a credit card-- the merchant fees simply cut too far into the dealer's margins.

However, the chances for getting miles seem to increase the less "price-sensitive" the buyer is. In other words, if you're willing to compensate the dealer for the merchant fee (either directly or indirectly), your chances of getting miles increase significantly.