I actually started gathering the overclocking figures very early today since there are no retail cards to add to the database yet. Out of the 19 reviews I found, (one was only a preview and one was about CrossFire performance) 12 of them featured overclocking results. It looks like the HD 7970 (average increase of 18.33%/12.445%) is a lot more overclockable than the HD 6970 was (average increase of 8.051%/7.284%). In fact, the HD 7970 overclocks better than any other AMD video card that I have ever posted a roundup for!
As you can see, Legit Reviews was the only site that managed to break the 1,125/1,575 (6,300) barrier that is imposed by the AMD Overdrive software. AMD Overdrive determines what the limit of the GPU and memory speed sliders should be by looking at the video card's BIOS. Legit Reviews was able to obtain an updated BIOS file from AMD that increased the slider limits to 2,000/2,500 (10,000).
According to Legit Reviews, AMD will provide the BIOS update to their customers as an executible (.exe) file. Hopefully the BIOS will be a little more stable by then.