All WP7 manufacturers (currently announced Dell, Asus, Samsung, LG, HTC) agree to have the following minimum hardware. Competition between handsets will be through design or improvements on the standard features below.
There is no developer access to the Start button, but the Start and Search functions can be made application specific
There is a future screen size with no ETA, 480/320 HVGA. This is to allow for smaller devices. There will only be two resolutions, and SDKs will have classes to detect and handle the differences in screen size.