Forum Moderators: open
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.
From Archive.org:
Apr '03
[web.archive.org...]
Feb '04
[web.archive.org...]
Jan '05
[web.archive.org...]
Feb '06
[web.archive.org...]