Here[0] is a link to a talk about wireless keyboard sniffing, including BT keyboards and their hardware/software solution for sniffing.

Specifically around BT keyboards they say this on one of their slides:

* Sniffing is possible but kind of “unstable”

* All pre-requirements for a successful PIN cracking can only be sniffed during pairing

* Complex documentation

So overall, not awful, but not fabulous either. Luckily most BT keyboards are severely limited in range (~ 30 feet max).

This is all from 2010, but is the latest source(s) I can find.

[0] http://www.remote-exploit.org/articles/keykeriki_v2_0__8211_...