I'm on a TN too. It's transferrable as far as I know because I've just done it (well I guess they just issue you a new one). I know the nightmare at the border fairly intimately. I always get secondary inspections. The second last time I got this whole giant grilling, with the officer threatening to not let me in. The last time the office said "I don't know why you're in here (secondary inspection), is there something you want from us?"