Likely in anticipation of more powerful hardware coming, Microsoft has lifted the restrictions on XNA code which prevented games from displaying at more than 30 FPS.
The relevant API update reads:
Windows Phone OS 7.0 behaviour
Windows Phone OS 7.1 behaviour
The XNA Framework on Windows Phone OS 7.0 targeted a refresh rate of 30 Hz, regardless of which PresentationInterval
The XNA Framework on Windows Phone OS 7.1 supports the native device refresh rate, up to 60 Hz. If you set the PresentationParameters.
PresentationInterval property to PresentInterval.One, then thePresentInterval.One value is used, targeting the native device refresh rate.
The native applications on Windows Phone 7 are all targeted to display at 60 fps, and this change should enable third party developers to create smoother-looking games also.
Read more at MSDN here.