Also, Bluetooth LE provides no eavesdropping protection. If an attacker can capture the pairing frames, they may be able to determine the "long-term key". Here's the NIST guidance paper on Bluetooth security: http://www.nist.gov/customcf/get_pdf.cfm?pub_id=911133

The attack surface can be minimized if the keyboard manufacturer implements crypto properly, requires encryption at the protocol level, uses a long and complex PIN, etc. The manufacturer with the best reputation right now is Microsoft. They got burned pretty hard when their propriety wireless encryption was hacked back in 2007, and it looks like their bluetooth keyboards are doing everything right.