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caveman - 5:52 pm on Feb 27, 2007 (gmt 0)
The last change I'm aware of is when they also started highlighting word forms, I think sometime back in mid-2005 (so if you searched on only a plural form, e.g., "blue widgets," they highlighted "blue," "widget" and "widgets"). But to be honest it's been a while since I paid great attention to the highlighted words in the Y cache.
The highlighting on searched kw's in Y cache has been there as long as I can remember.