I was absolutely thrilled when I saw the specs for the 27" iMac, but when my Quad core enthusiasm settled, I began to think about the one mistake Apple have made with this otherwise amazing machine. They committed the one mistake that feared they would, they neglected the GPU 
The 4850 is a fine card for something like the consumer 21,5" iMac, or the prior 24", but I seriously fear that the 4850 will come short when it comes to throwing around HD 3D content on that gorgeous 27" screen
I know you have little chance of answering this question, but do you think it would be possible for a person with technical skills to, swap the 4850 with a future 5850, if Apple should choose to implement in future iMacs. Will it be theoretically possible
I don't expect it will, but I hope for a miracle
The 4850 is a fine card for something like the consumer 21,5" iMac, or the prior 24", but I seriously fear that the 4850 will come short when it comes to throwing around HD 3D content on that gorgeous 27" screen
I know you have little chance of answering this question, but do you think it would be possible for a person with technical skills to, swap the 4850 with a future 5850, if Apple should choose to implement in future iMacs. Will it be theoretically possible
I don't expect it will, but I hope for a miracle