I am reading up about linking native libraries into MonoTouch, specifically this documentation:
http://monotouch.net/Documentation/Linking_Native_Libraries
Here
You can certainly use a C++ api, but you will need to either wrap it in a C api, or a ObjC+btouch api to be able to talk to this. Invoking C api's uses PInvoke, and C++ libraries use symbol mangling. While theoretically you could pinvoke to the mangled symbol, this isn't a good idea as the mangling is compiler specific.