For those using Windows Phone 7.5 but still looking to use Google as their search engine, Google has released the Google Search app for Windows Phone. Google Autocomplete, My Location, and Voice Search are all supported.
Whether you really need the Google Search app is up to you. It basically offers the same functionality the Google search website. The only addition is the Voice Search capability.
The Google Search app for Windows Phone is a free download for Windows Phone 7.5 devices. It is available from the Windows Marketplace.
Read more: Google Mobile Blog