No - Carmack said they will patch-in the support. Make sure you are here when the game ships and bring up those Eyefinity threads.
Found some info. Carmack said he might play with Eyefinity after Rage is released and then send out a patch that enables it. Doesn't sound like a sure thing though. I'm canceling my order for now.

. Meantime, I'll play Deus Ex Human Revolution which is released with native Eyetinity support.