Most self-serve AvGas pumps have qualified for the Amex rebate for me. Can't say if they're supposed to or not, but they have. Most AvGas from an FBO truck paid at the desk, has not. I suspect it's how the point of sale is encoded in their database.
Note: Most of these rebate deals have a hard cap on them at a certain dollar amount on fuel purchases. Doesn't take too many airplane fill ups to hit the cap. Once there, the rebate isn't useful anymore. Make sure to monitor the cap and switch to some other card that'll give you something for nothing.
Other evilness seen lately, gas stations that change the fuel price at the pump higher for credit card purchases vs debit card or cash. Card companies didn't used to allow this, but it looks like they either don't care anymore or they're not paying attention.
And of course, keep in mind that Amex charges businesses significantly more than Visa/MC. Don't whip out the Amex for perks at your favorite small business if you know the owner and like them.
Seeing a trend toward small businesses just refusing to do business with Amex around here.