I would consider the 5xxx and the 3xx the same generation, obviously they're the 1st cards to handle dx11 from the both companies, they both are built on 40nm tech, we could have been anticipating the 3xx in late this year if it didn't have any production issues, and the delay in releasing the 3xx doesn't mean it will be benefiting from better technology as pointed earlier by OvrClkr.
A little off the point, but i'm a little sad about the 5870 as well, this card should theoretically have 2x the performance of the 4890, but we see the 5870 fluctuates between around the performance of the 4870x2 (which is around 170% the performance of the 4870, let alone the 4890), we saw AMD doubling everything (shaders, rops.. etc) except for one thing it wasn't doubled, the memory bandwidth.
This leads me to think that the 5870 is bottlenecked with its own memory bandwidth, of course how much the card is bottleneck depends entirely on the way the application is coded, for me that's why we see the 5870 lags behind the 4870x2 who has the same specifications on papers but with increased bandwidth, although it suffers from scaling issues, and and surpass it in other applications that doesn't depends largely on memory bandwidth.