Some Radeon 3850s will work at over 700MHz. The default clock speed is 670MHz, but for the first time in history, ATI decided to let its partners go wild with the clocks, with some limitations in mind.
At least some AIBs will launch cards that work faster than that, and they will go significantly over 700MHz. We are sure that the same people will go over 800MHz with 3870 cards, but this is something that has to be confirmed.
Finally, you will have a real noticeable difference among the cards, as some of them might be faster for only a bit more money.