Newegg Randomly deactivates some of the ati x2 cards, I think they been ordering in low numbers and some days they run out and other brands come in. I been keeping an eye out been tempted to get an open box 4870x2, I been watching it for a few days most of the time if they have one it's a sapphire or an asus tri cooler then once in a blue moon you will see a powercolor or a visiontek open box.
For regular cards it's random at the moment, I noticed not too long ago they added xfx cards.
Honestly I don't think they will start making a 4890x2, it would just add more cost to make with little yields.
I hear the next ATI generation card might be coming out by this summer anyways.