Well if you think about it, from the pictures you can see the card is the length of an ATX Mobo + about a centimeter and the Antec 900 has a good inch or so between the mobo and whatever is there (HD's I think) so unless you are running loads you could just move them out of the way, also with the power cables now going in the outside (eg like Nvidia do now) there is no reason for it not to fit.
Your right. One thing I read, but forgot about was the power cables going in from the side. That will help a lot.
Looking forward to some reviews now. If a 5870 can match a 4870 x2 then I think I might not be able to resist. I was tempted to get a 4870 x2 once I got my 30" Dell, but I would have needed a new PSU and new Mobo, as my one is not PCIE 2, and to be honest I am still able to play most my games at max res with the single so I decided its not worth it.
I am looking forward to Mass Effect 2, and would love to play that at 2560x1600. So will probably need 5870 for that.