My guess would be early 2011 release, Dual Cayman (most likely Dual HD6950's or Dual HD6970's underclocked). 12-13" in Length, 300W Power consumption, 30-50% faster that HD5970, Dual slot (Obviously), £550ish. Hotter than the sun.
(Just my guess, not facts....)
From my experience so far, the 6xxx series is slower than the 5xxx so my opinion would be it will be within +- 5% of the 5970 and VERY similiarly priced, with a few eye catching features