ClickZ reports that they have noticed when using Microsoft's new browser, Internet Explorer 7, and visiting Google, you are prompted with a DTHML popup box that says, "Make Google your Search Engine in Internet Explorer," as you see here:
Is this Google's way of fighting back against their objections of IE7? Google is upset that Microsoft sets MSN Search as the default engine on IE7. So if you are using IE7 and visit Google, Google will go out of their way to help you switch that default to engine to Google Search.
This is not the first time Google or even Yahoo has promoted their search engine. Philipp Lessen reported this exact thing for IE7 users back September 2005. Also, Yahoo did the same, to IE 7 users in January 2006.
Postscript From Danny: I added the Google image above, plus Yahoo's still doing the same thing, as you'll see below: