Nothing at all and Everything in general.

What to Expect from XNA Game Studio 4.0

XNA Game Studio will be made public within the next one to two months. Here's a few of the updates

  • You'll be able to use the Mouse APIs on Windows Phones. The Mouse API will actually use the Multitouch functionality and return the position of rhte first point.
  • There are two XNA Profiles, Reach and HighDef
    • The Reach profile prvide maximum compatibility across the three screens
    • The HighDef profile is for Xbox/PC. While it lacks WP7S Compatibility it lets you take advantage of the device's capabiities
  • The Multitouch APIs are now available to Windows targets
  • The ZuneHD won't be getting XNA 4.0
  • XNA 4.0 isn't designed to be backwards compatibile with XNA 3.1. Compatibility was sacrificed in the interest of progress.
  • Windows Phones will have the Same aspect ratio but different screen resolutions
    • To the developer both devices have the same resolution. WP7S contains hardware scaling
    • WP7S also contains hardware functionality for scree rotation
  • Custom effects are not available on Windows Phone 7 Series
  • Emulator will be available. You don't need to own a Windows 7 Phone.