When Nokia revealed its flagship Windows Phone 8 devices few weeks back, they mentioned that both the devices has Bluetooth 3.1 built-in. Even now, product specs on Nokia website confirms the same. But I think more good news is on the way for Lumia 920 and Lumia 820 fans as both the devices got certified for Bluetooth 4.0 specs.
Bluetooth 4.0 introduces support
for collecting data from devices which generate data at a very low rate. The main intent of this feature, called Low Energy (LE), is to aggregate data from various sensors, like heart rate monitors, thermometers etc. Bluetooth was one area where Lumia 920 fell behind in the specs when compared against iPhone 5, Galaxy S III,etc,. And it may change soon!
Source: Bluetooth.org (1)