Highlighting search results by relevance

System_preferences_search
I was playing about with the search bar in OS Lion today when I noticed a great way to indicate the relevancy of the results. In the example shown, I searched for the word 'dock' and as I typed the first 2 letters a number of icons were greyed out, leaving only a few slightly highlighted by a spotlight.

Then, as I completed the word 'dock' the icon for that feature was more obviously highlighted and the parental controls feature was ever so slightly highlighted. This was an excellent way to visualise how relevant different search results were. I could easily discount the items that weren't highlighted and I could clearly differentiate between the strength of the highlights.

Can you think of any other UI's that could use this feature? Why haven't we seen something like this more often?