[goodness]

Actually, their docs claim that the limits aren't really related to the API key. The API key is just so they can contact you if they start blocking you:

  http://code.google.com/apis/ajaxsearch/key.html

That may just be disinformation though. For example, if you can spread your usage over multiple IPs, you might be allowed more searches than if you told them your API key for all your requests on multiple IP addresses.