Even if they had this stuff stockpiling in storehouses, they wouldn't release it till after the holidays in order to not confuse the x-mas shoppers. The chip roll-out is gradual starting from high- to low-end or from mobile to desktop. They can't do that in two weeks at the end of the year. It's also a different architecture, which would make driver support for both old and new cards really hard, after the current flood of game releases...
Q1, 2012 some say Q2, 2012, because their Radeon 7000HD rumors turned out to be some 40nm re-branded mobile cards. Past three generations have been released June-November.