I'd say 30mh is really all we're going to get on the 480, but 2 of those for $400 getting 60mh @ 300W is still a pretty good deal. I'm considering picking 2 up, but want something to hold me over for the next month. I'm going to see if my microcenter has any 380's and if they'd be willing to take them back for the 480 when it launches.
It will be less than 30 MH/s. The memory bus is just 256bit, it is lower than the 512 used by the 390.
Memory is also a lot faster - 8 GB/s vs 5GB/s or some such bandwidth figure.
The increased speed should make up a lot for the narrower bus, and archetecture upgrades *might* make up the rest - or might make it worse - THAT is the real wild card we can't judge without a card in hand.
All indications are that it shoud solidly beat the 280/380 and probably the 280x but not sure vs. the 290 or 390.
i would guess a right estimate around 25MH at a lower wattage than their tdp, like it was for eveyr other gpu
you cna expect to maintain the same hash at 33% lower consumption, this only with etheruem because of the low memory usage
? Cooler blower is so loud. 
!