AMD hasn't officially commented on when the Radeon HD 7000 series support will land, but based upon the time that's passed already and some other activity, it's hopefully coming soon. Even when AMD launched the Radeon HD 7950 and then the Radeon HD 7570/7770 graphics cards there was no open-source support. Right now for Southern Islands there is just backing from the AMD Catalyst blob, which remains frightening to many desktop Linux users.
This afternoon Alex Deucher published a new Radeon DRM driver patch-set entitled radeon_asic cleanups for drm-next. This "clean up" is adding in over 1,200 lines of new code while removing just over 700 lines. There is no change in functionality with these 14 patches that were presented today, but there are changes "required for future functionality" and other cleansing.
This would be a prime opportunity for AMD to now publish the Southern Islands support for their Radeon DRM kernel driver so that it can get sufficiently reviewed and ready for the Linux 3.4 kernel merge window. (If the new hardware enablement doesn't require any other changes that could regress older hardware generations, it could potentially be mainlined at any point, but based upon the VM changes and other work the S.I. undertaking is quite large.)
Besides needing to address the Radeon HD 7000 series support within the Linux kernel, there's also the Mesa/Gallium3D support that's needed. Due to the new GPU architecture, AMD simply can't tack on the Southern Islands support to the existing "R600g" Gallium3D driver but rather they need to make a new Gallium3D driver. This new driver is said to be based upon a stripped-down R600g, but still it's taking a while and will also need to clear AMD's legal review, which can be a very lengthy process.
A small update is also needed to the xf86-video-ati DDX, but these days that's minimal with AMD no longer providing user-space mode-setting (UMS) support for new hardware.
Soon as I hear more about the Radeon HD 7000 series open-source Linux driver support I'll pass it along, or when any Southern Islands samples finally arrive for Linux testing with the blob or otherwise.