If you open the Web Inspector and press Shift + Tab, the elements of the inspector take on an unfocused appearance and the placeholder text of the search field is editable.
Created attachment 17948 [details] Screenshot
Hrm, doesn't appear to happen on Windows.
Created attachment 18019 [details] Test case
This happens in any search field it seems, not just the Web Inspector.
I can not reproduce this with the test case (using stock 3.0.4 or r28774 on Tiger). If I have toolbar enabled, shift-Tab just moves focus to Google search; otherwise, it stays in the search field.