Looks like Gmail added a new feature!

You can now control “highlighting” of searched words in Search results!

When you initiate a Search, Gmail returns a list of any messages and conversations matching the Search criteria. You can select any of these messages or conversations to view, and Gmail automatically highlights the word(s) you searched for. My sometimes, it’s distracting, so you now have the ability to click on the “Turn off Highlighting” link on the right column. Clicking it again turns highlighting back on. Very nice! And like just about everything about Gmail’s interface, it’s lightning-fast. No need to wait for the screen to re-draw like in most other Web-based email services.