Does anyone know if there are any compatible Wireless-N/MIMO/802.11n (miniPCI or other) adapters that work with Router OS?
Not yet. There is a number of pre-N products that are “in development” but nothing yet. To be honest, I don’t think it is going to be good tech, and also, the last I read, I don’t think they are planning on ratifying the standard till 2009.