Both of them seem unlikely. Northern Islands (Cayman) was originally supposed to be on 32nm but due to the cancellation of it by TSMC, AMD resorted to 40nm, which still seemed to work fine. It's hard to say if Cayman would have been faster on 32nm, due to less power consumption and therefore more shaders, but probably more expensive.
Which basically means that, how far they were into 32nm research/production when it was cancelled for 28nm is probably the same amount of time between the original planned release of Cayman and the original planned release of Southern Islands (whatever it was). Then there's the approximate 6 month delay between new GPU's as well on the side (I'm talking a wild guess) but it still seems too far for Q2 2012. Q2 2011 seems more plausible because it wasn't AMD's fault, they're not the ones behind. But maybe they took this opportunity and that lost time to improve their next GPU. But yeah, Q2 2011 isn't possible, but neither is Q2 2012.
I'd say Q4 2011, or late Q3 2011 if we're lucky, for 28nm GPU's from AMD.

