Windows 10 IoT Core supports Bluetooth 4.0. A list of supported Bluetooth dongles can be found in the Hardware Compatibility List.
IoT Core supports the following Bluetooth profiles:
Human Interface Device Profile HID
Radio Frequency Communication RFCOMM
Generic Attribute Profile GATT
When using one of the Windows 10 IoT Core Release Image Bluetooth devices can be paired with the Windows IoT Core device using the device portal. When navigating to the Bluetooth tab the device will look for Bluetooth devices and will also be discoverable to other Bluetooth devices. The picture below shows an incoming pairing request.
After the device is successfully paired it will be listed under the paired device section