The Bluetooth SIG has posted the updated Bluetooth stack for Windows Phone 8.1 Update 1.
While Microsoft touts the main new feature as being Bluetooth Low Energy support, the updated stack also adds Bluetooth Personal Area Network (PAN) support, which will can be used for internet sharing and also for supporting smart accessories like smart watches.
Compared to Windows Phone 8.0 the changes are a lot more dramatic. That stack dates back to August 2012, and only supported the Handsfree Protocol 1.5, vs 1.6 in the latest version. Version 1.6 adds support for high quality audio via the mSBC codec, which should mean with supporting devices better sound quality and voice recognition. It now also supports Voice Recognition Activation, which should be handy with supporting car kits for Cortana.
Microsoft has recently specified that OEMs can no longer extend the stack, like Nokia did to add Bluetooth LE support in 2013, meaning for now we have to rely on Microsoft to keep the stack up to date.