Poor Agent Barrett, ever the dumbass in a suit who's rule sticking clags up the works. And now he's still all that but he's also a brainwashed agent of the enemy. I wonder if we'll see him again.
I know you're generally against spoilers, but you've already watched Atlantis, so you already know the answer to that.
But a hint: season 10 SG-1 corresponds with season 3 Atlantis, meaning seasons 4 and 5 Atlantis are post-SG-1.