This may not be that big a leak but, if any doubts were there in minds of anyone after Microsoft store removing Lumia 635 listing for AT&T then that should get cleared. evleaks has now posted render of Lumia 635 with AT&T logo on its back.
Microsoft had listed AT&T bound Lumia 635 for just a price of $99 as compared to $129 pricing for T-Mobile version, so it may be less expensive at AT&T, as we have seen in the case of Lumia 520 and Lumia 521. In the meantime, T-Mobile Lumia 635 has the best price at HSN for its No-contract version.