T-Mo has tweaked the search within Internet Explorer only so that when browsing the web and you push the search button, it takes you to the T-Mo Favourites page with a Google-powered search box on it. That behaviour only occurs when you're currently browsing a page.
If you're not already browsing, and you push the search button, the standard Bing Search page will appear.
Also, bear in mind that you'll get 3GB of browsing a month with T-Mobile as opposed to 1GB with 3 or 500MB/1GB with Orange, and T-Mobile isn't quite so aggressive as other networks if you exceed your browsing allowance. Generally, if you exceed your allowance by a modest amount, they'll let it go; if you exceed significantly, they may throttle your speed; if you persistently exceed by a significant amount, they'll send you a letter or give you a call to discuss your usage.
Having the T-Mobile redirect to their search page from within IE is a small price to pay for such a generous web allowance - and if that redirect really bugs you, there's a very simple workaround: just remember to press the Start/Windows button before pressing the search button. That will take you to the Start screen, and then launch the full-screen Bing Search page. One extra button-push for an extra 2GB a month is worth it.
The only other customisation that T-Mo have included on the phone is their logo splash screen, but the phone boots up so quickly that it doesn't really make much difference having it there. I half-expected them to customise the OS colour scheme with white background and pink/magenta tiles - but it was supplied with the standard dark background/blue tiles theme.
So yeah, T-Mobile every time. Hope that info helps.