I don't do as much tanto dori as I probably should. I'd say one important thing is to remember that your enemy in that situation is the man wielding the knife, not the knife. This is not to say that you ignore the knife and go for broke...just don't get so caught up with the blade that you forget the man behind it (or his friend behind you).
I've seen some of the work done by people who specialize in knife work. I'm not completely convinced that standard aikido techniques done against standard aikido knife attacks are sufficient in preparing to face a knife specialist. Better than nothing though.