There's pros and cons to either interpretation. I think I'm siding with immune to AMF, just like Wall of Force.
My PC's will soon be hunting down a McGuffin (a Weirdstone) in order to obtain a more important McGuffin (the legendary sword
Gram*). Due to the situation, the patron that has Gram and is sending the PC's on the quest for the Weirdstone, he knows where the Weirdstone is (Discern Location), but not who or what has it. That creature is actually a gestalt Concordant Killer//Sorcerer 19, with Mindsight.
I'm going to turn my own PC's tactics against them a bit (they recently snuck into a "Roman" military outpost and murdered the mage governor there by using Time Stop and eight Delayed Blast Fireballs, plus stoneshape to seal off the door to his office, along with a readied Dimensional Anchor to stop him from teleporting. I did incorrectly let them roll their own time stop duration, though; it's supposed to be unknown to the PC), because they are good tactics. The Concordant Killer will have Time Stop and Delayed Blast Fireball; that along with his 3/day Forcecage SLA, and the activated Weirdstone, and he'll be a tough foe for a 19th level gestalt party. If they aren't careful, he has a decent chance of trapping them when they come (though weirdstones don't actually block Shadow Walk, so the Beguiler should be able to get them out, if they are trapped together.)
Not that any of my three arcane-casting PC's have AMF as a spell known or in their spell books; at least, I've never seen them use it. But the Concordant Killer will have it as a spell known, so determining how they interact is important.
*
I've given Gram the following stats:
Gram is an Adamantine +5 Berserker Dragon Bane Greatsword. Gram inflicts the effects of Graymantle (SpC p. 107) upon any dragon that it strikes; the graymantle effect lasts one round, and no saving throw or spell resistance can resist this effect. In addition, a wielder can formally claim Gram as his own by grasping the hilt and over the course of three minutes (consecutive full-round actions, uninterrupted by any other actions) willfully expend 1,000 XP. At any point thereafter, the wielder may call upon a surge of fortune as an immediate action, treating his next attack roll with Gram as a natural 20. (The attack roll must occur within one round of invoking this power, and you must still roll to confirm the critical hit normally). No one else can formally claim Gram until the wielder expends the surge of fortune, or he dies, breaking his link with Gram.