This is a technique I've seen applied to bulletin boards, but never on eCommerce sites, and I'm not sure why. Really, I'm not sure. So you have an eCommerce site, let's call it something cliche like "DiscountWidgets.com." A user types the query "inexpensive macintosh compatible widget." He sees a SERP that's highlighted with relevant-looking text bolded in the excerpted copy, but he want to see more.  That's nice, but when he gets to the page, it's not bolded anymore, so he's lost. If he's not totally determined to find what he's looking for (and most users aren't), then he proceeds to hit the dreaded back button and you've lost your lead :(.

So why not use HTTP_REFERER to grab their keywords and highlight them in bright yellow?  That way users know where to find the relevant content once they arrive.  It may be wise to tokenize the keywords as well and highlight individual words if the full phrase cannot be found in the copy.  That way it won't just find and highlight "inexpensive macintosh compatible widget," but also "This widget is inexpensive, and compatible with your macintosh."

If you're really motivated, you can also implement a stemming algorithm so you recognize "macintoshes" as an inflected form of "macintosh," and highlight that too.

And, while you may assert that the user can use the search function in his or her browser, or that certain toolbars already do this, how many users don't even know which search engine's toolbar they use, let alone change their search engine (deliberately, in either case, anyway).

It's a trivial matter to do this (except perhaps the stemming support), and I will offer code upon request.  I do it on Lawyer Seek.

So a user types in "accutane litigation," clicks on the result, and the "Seeger Weiss and Accutane Litigation" text on the page is highlighted in yellow. This results in a better user experience, and the user is more likely to convert to your customer or client.