I found a link...
http://www.mitsubishicars.com/MMSA/GasCardOffer
Most of our questions are answered in the fine-print disclaimers at the bottom.
It seems to imply that they've made deals with several gas companies to provide a gas card. You have to pick one of the "qualifying" companies (so you can't buy your gas just anywhere).
They also indicate that this can't be combined with any other offers -- so it's not over-and-above other common things like rebates and financing deals, it's instead-of other offers -- you have to pick what you want... cash-back rebate, lower interest rates if you're financing through them, or the gas card.
I still don't understand how they establish the specific value they intend to credit to your card. Is it
(12000 / you cars EPA fuel economy rating) * $2.95?
Do they adjust it based on actual average price of gas (so $2.95 is just a cap -- when gas is $2.65 they pocket the 30 cents)?
If it is based on EPA, then it is city or highway rating or something in between?