ATI’s next generation Southern Islands cores are not promising anything spectacular. It looks like these cores are going to be just a tweak of ATI's existing Radeon HD 5000 series architecture.
Sources:
Radeon HD 6000 is a minor improvement - Graphics - Fudzilla
ATI Southern Islands / HD 6000 a minor improvement by VR-Zone.com
Probably not enough to beat Fermi.
But let's wait on how flexible ATI's RV870 arch is and how much performance they can tweak out of it.
nVidia has already proven the flexibility of Fermi with GF104. This might be their chance to get back on.