The three buttons are a back button, a Windows button, and search. Those three buttons, combined with the OS in Windows Phone 7 Series, help to create a great user experience, and that is one area that Microsoft is placing heavy emphasis in. According to Loke-Uei at the Lobby Lounge at the Microsoft stand at GDC, the search button is context sensitive, meaning that if you hit the button in your contacts application, it will search for relevant contacts. With the HTC HD2 having 5 buttons, 3 of the 5 can be mapped to the back, Windows, and search buttons, but being that the OS is targeted more at consumers, it may create some confusion since those buttons are already hard-labeled on the HD2 for their functions right now.
Additionally, although the HTC HD2 meets the screen resolution down on a capacitive touchscreen as one requirement for Windows Phone 7 Series, Microsoft still has not stated what the exact CPU, GPU, and RAM requirements would be. Microsoft is creating standards here to make it easier for developers to build apps and games for Windows Phone 7 Series, so it makes sense to not only have common APIs, but also have similar hardware. In the video below from part of a GDC session, Microsoft is stating that OEM variations may come in the form of a hardware keyboard. This way, with similar other specs--RAM, memory, processor, and graphics--the Windows Phone 7 user experience--an area that Microsoft is stressing--will be similar across all makes and models of phones.
We'll try to get an official answer at MIX10 or at CTIA in a few short weeks and let you know what Microsoft and HTC will announce for the HTC HD2, but for now it may make the most sense to keep a unified, simple, and consumer user experience
