Windows Phone 8.1 Developer Preview coming first part of April, tweets Joe Belfiore
Any new feature coming to Windows Phone 8.1 handsets via the Nokia Cyan update, will not be available in the Developer Preview. For example, the hardware drivers that will allow the Nokia Lumia 1520 to support Miracast, will not appear on the Developer Preview, and those waiting to use the screencasting service on their Windows Phone phablet will have to wait for the official release of the new OS build this summer.
Belfiore also revealed that Local Scout, which gives information about nearby stores and eateries, is now available via Maps, and that Bing Music will be part of virtual personal assistant Cortana. He also stated that the Music app will be upgradeable from the Windows Phone Store. This means that Windows Phone users will be able to obtain updates for the app by merely downloading it from the Store, instead of waiting for an OTA update to be sent out. The process is similar to how Motorola allows System apps to be updated in the Google Play Store. There is talk that the Windows Phone calendar app will also be store upgradeable.
source: @joebelfiore (1), (2) via WMPoweruser