My theory on all this is that ATi got their pants pulled down by Nvidia this generation when NV announced they were doing 16 pipes, as ATi it looks like was planning on 12.
Think about it - X800Pros never had availability problems, but X800XT and XTPEs - ATi's 16 pipers - never really came out. NV on the other hand shipped 6800GTs by the boatload, and 6800Us showed up in a couple months. ATi's XT and XT PE however, you still can't buy, not even now.
To me, from a distance, squinting, it looks like NV planned the 16 pipe jump all along, whereas ATi was moving it up to 12, and tried to counter swiftly by bolting some pipes on to their core. Unfortunately, yields seem to be a problem.
This is all conjecture of course, and I could be completely wrong.
Anyway, the proof is in the pudding, so to speak. ATi is having volume trouble whereas NV - and bear in mind Nvidia's chip has a SIGNIFICANTLY higher transistor count than ATi's - has been shipping in bulk for several months now. Whatever the reason, one company clearly has the advantage at the moment.