Granted that we’re not surprised with Bing being the default one in use by the platform, you can actually change it to Google under the IE9 settings menu. For long time user of the platform, this might be seen as a foreign concept – especially when Bing is ingrained deeply with the mobile platform. Nevertheless, it’s still nice giving users the opportunity of which one to choose.
So far, there’s no exact reasoning as to why this option is only available on the Mango flavored HTC HD7 for T-Mobile only. Still, the rest of us sporting Mango will have to make due with Bing as the primary and mandatory option for now.
source: WP Central