Re: No defence?
Ma-ai, distance, is a good defense against a knife. My hand-to-hand combat instructor in the army used to say that you run from a knife and charge a gun (you can't out run a bullet, so goes the theory). But, if you have to get in a knife fight, expect to be cut, just try to make sure you are not cut in a vital area before you manage to take down the other guy or make your escape....
As for an aikido defense against such an attack, I think it is good to remember that while aikido is generally characterized as a "defensive" martial art this does not mean that you just have to stand there and wait till some aggressor makes an overt move against you. Aikido training also has to do with observing your surroundings and other people. If you perceive an imminent attack, nothing in aikido says (just the opposite, in fact) that you can't do a preemptive strike of your own against your attacker.