There are some security holes with Bluetooth. The fairly recently released v2.1 made some improvements. I've not yet designed a Bluetooth keyboard so not exactly sure what the security issues will be with it specifically. I'm assuming most Bluetooth keyboards would use the HID profile, which I'm not presently familiar with. In thinking about it only very quickly, I can see two possible security issues.
1) If someone were able to eavesdrop on your connection and capture all your keystrokes, they could obtain some sensitive information.
2) If someone were able to mimic your Bluetooth keyboard, and if your computer was unattended while you were logged in, they might be able to type some commands to retrieve sensitive information on your computer. However, I think this is extremely unlikely, due to they wouldn't likely be able to view your screen while doing this.
I don't think the second issue needs much addressing being the likelihood of someone pulling this off is very small and it can easily be prevented by not allowing your computer to sit unattended.
For the first issue, like I said, I have no real experience. If you want the keyboard to work in a standard fashion though, such as able to connect to a Mac as-is, then I imagine you don't have any options in making the connection any more secure than what Apple designed. I did once design a Bluetooth product where I had complete control over both ends, with both ends being run on microcontrollers. I designed my own proprietary Bluetooth stack, encrypted the data being transmitted, plus the raw data was just a bunch of numbers and some control fields. Pairing between two devices was fixed in the lab before the units were deployed and new pairing was not allowed. There were also a few other security safeguards put in place which I cannot mention. So I believe in that case the system was pretty secure.
If I was to attempt to sell commercially my keyboard controller, and I thought security would be a concern of some of the buyers, I'd probably do something similar to the previously mentioned product. Being my controller would be targeted at a high-end keyboard, the cost of the controller wouldn't really be an issue. So building my own proprietary Bluetooth stack to run on the micocontroller would be easy. For a mass-marketed keyboard, it's likely not so feasible, being they'd first have to find a firmware engineer capable of designing their own Bluetooth stack (not so trivial), and need to probably use a more expensive microcontroller than what they intended which had enough memory to support the stack. Add to that encrypting the data, and secured pairing, and I imagine it would be secure enough to keep out all but the most determined and capable hackers. Of course I'd also need to support the standard HID protocol for users of Macs, etc. where they might not be able to or want to add the keyboard's customized Bluetooth dongle. In which case the user would have to choose between the more secure connection or a less-secure but standard connection.
Anyways, I'm not giving too much thought to the security at the moment. I'll cross that bridge when the time comes.
