Ok, just went down to the Church in Shoreham to take some test shots inside between the 1ds3, 5D2 and 5D3.
Based solely on IQ (I don't touch video) the 5D3 is essentially a 1Ds3 with higher ISO capabilities.
Versus the 5D2 it's a fair old jump, colours are represented better and more accurately.
Noise still gets nasty beyond 6400 ISO but that's because I never touch noise redution.
The noise that is there is more grain like and less colour based.
The files themselves (jpegs) seem more robust to edit and generally have a smaller file size overall (due to cleaner files).
Dynamic range is certainly stable all the way to ISO6400, on the 5D2 the cost of high ISO would be a fall off in vibrance, not so on the 5D3.
Other than the above, feels great in the hand, changing the zoom buttons...bad Canon, silent shooting is just something you have to hear to be believed.
Overall, a wise purchase if you're going to use it for the 3-4 years between models..but even then I think this model illustrates we are nearing the realms of sensor limits. You know, the IQ isn't what I'd consider a 4 year jump and I very much doubt this will be anything more than a backup for the 1Dx.
The only caveat to the above is that the official, non beta RAW convertor isn't available for Lightroom, which is where I do all my processing. I would rather of shot RAW.