A Google Web Search Help thread has confirmed reports of a bug with the Google Cache page.
Typically when you search for something and then click on the cached page, Google will highlight the word you searched for on the cached page, if it is on the page. But now, if you are trying to see the highlighted results you may not get it.
If you are signed into Google when searching, Google will not highlight the word. If you are signed out, Google will highlight the word.
I assume this has to do with the SSL search dropping query data to pass to Google's cache. But Googler, Sarah, made it sound like it is a temporary bug.
Thanks all for your detailed responses in reporting this. This is something that we are aware of and are looking into.As you have found, for now, if you want to see the terms highlighted, you can do so by signing out.
Screen Shot of Highlighted Words When Signed Out of Google:

Screen Shot of No Highlighting in Google Cache When Signed In:

Again, I suspect this is related to the SSL change.
Forum discussion at Google Web Search Help.

Comments:
Cindy H
12/16/2011 01:56 pm
I really appreciated reading this information. I use Google a lot, and I just couldn't figure out why it wasn't highlighting my search words anymore. Sure enough, as soon as I signed out, I was able to see the search words highlighted! Thanks!
Rusheggie
03/16/2012 06:01 pm
thanks, worked for me too
Guest
04/28/2012 08:24 pm
I'm signed in, still no highlighted words when I search. Is there an add-on? Please advise.
Cindy
04/28/2012 11:44 pm
Twelve. No, the point is to sign totally OUT of Google, not sign IN. :o) Good Luck!
Jefferson Scher
12/17/2012 01:34 am
After waiting more than a year... I wrote a userscript to resolve this issue: Google Cache Highlight Search Query Terms for HTTPS http://userscripts.org/scripts/show/154167 It's still in testing, so feedback is very much welcomed.
Cindy at JBK Pottery
01/10/2013 05:20 pm
Thank you Jefferson. I'm such a slacker, it's taken me this long to load (is that the correct term?) :o) your userscript. It works beautifully, and I thank you for it, since I search a LOT on the internet for work. Appreciated!