Considering the close heat race that ATI and Nvidia are experiencing right now I expect that neither will chose to "wait and see" on their releases. While, I agree that it's likely that the dates may slip due to technical reasons, their is little reason for either to delibrately wait on pulling the trigger.
Even if the dx10 capabilities have no value, these cards will still be a significant step up in capability for dx9. The Oblivion benchmarks, show that this performance increase has value.
If Nvidia, for example, makes the decision to delay, and ATI does not... ATI will sweep up the highly profitable premium market.
Additionally, it is expensive to tool up parts, and then not run the machines... for those two reasons (and a host of other business type reasons) I doubt that either will wait for Vista.