Their reasoning behind the assumption lies in the frequencies supported and the model number of the phone. This is what MobileCrunch had to say:
"As far back as we can remember, Samsung models beginning with SGH and ending with 7 (like the Propel [SGH-A767], Propel Pro [SGH-i627], Mythic [SGH-a897], Impression [SGH-A877], Eternity [SGH-A867]) have all gone to AT&T. Couple that in with the fact that it’s a GSM handset with EDGE bands (3G radio bands weren’t disclosed) that play friendly with AT&T, and it’s all just coming together too perfectly."
So if this turns out to be true, we just have to hope that by the time the Galaxy S gets released, AT&T will have changed their stance on how they limit what Android phones may do.
Image from Engadget