Let me make a clarification.
The problem is real, and we got it with the unique URL bar behaviour in Safari 6.
If you make a google search from the URL bar, Safari doesn't bring the focus to the document, but (for some reason, e.g. assuming you continue with typing new search keywords) leaves the focus in the URL bar. This causes that you can't instantly use the very advanced keyboard handling of Google's search results pages - which is: from the input field TAB gives you a selector arrow and you can navigate with keyboard arrows.
For using the search result page's keyboard handling, you have to bring the focus to the document. Now, whether it's easy or not depends on your keyboard settings in System Preferences. If the setting which says "Full Keyboard Access: In windows and dialogs, press Tab to move keyboard focus between:" is set to "Text boxes and lists only", then it's easy to bring the focus to the document by hitting Tab only once. But if it's set to "All controls" (which actually happens a lot if you're accustomted to using the keyboard in UI navigation), then Safari brings you through all the bookmark bar items and such buttons when you hit Tab. This is definitely a no-way.
You always need to switch back to "Text box only" (by hitting Ctrl-F7). The problem is that in this mode you can't navigate between form elements like checkboxes and buttons.
In Chrome the problem doesn't exist, because Chrome always brings the focus to the document when doing a search.
Apple should definitely address this issue.
Until then, a workaround can be that you hit Ctrl-F (moves focus to in-page search), then hit Tab 4x.