That is exactly what i was wondering.
My thoughts:
6xxx series are 40nm, improved core design on 5xxx, but no extra cores. 6970 is a 5870 with upgraded cores, 6990 is a 5970 with upgraded cores.
Come 28nm time, those same high efficiency cores are doubled in number (guesswork, but that seems to be the plan these days) and the naming moves on to 7xxx, with the usual 7870 being the top single gpu (possibly 7890 later ala 4890).
I can't see how they could fit the 28nm stuff into 6xxx.
EDIT:
Given that the 28nm core has already been largely developed and mostly what ATI/AMD is waiting on is TSMC's 28nm process to be worth using (40nm type issues do not give good press), i think 6xxx will be a very brief operation.
Alternatively, 6xxx could stay a while as the replacement for x7xx, and the high end stuff will all be 28nm on 7xxx names.