Just want to make sure I have this right:

If I place this on an opponent's character,<edit> assuming it is a sandsnake or I make it one</edit>, and it becomes immune to all of the character's controller's non-plot card effects...?

Per the FAQ:

"Opponent's character abilities" refers to
any ability on a character controlled by an
opponent. Abilities on characters you control
are considered your character abilities even if
they are triggered by an opponent.

When a card refers to an "opponent," it is
always referring to opponents of the player
controlling the card making the reference

And the card does not state that the character gains immunity, just that it *is* immune.

Am I missing something obvious? Or am I just the last to realize this? (Yeah, that happens a lot!)

-Pedro

It all comes down to this: the attached character is immune to the non-plot card effects of the opponent's of the attachment's controller. If a different player controls the character than controls the attachment, the character is still immune to the non-plot card effects of the opponents of the attachment's controller. And yes, that will usually mean that the character is immune to the card effects of its own controller.