There are sooooo many different opinions on this. Each mother should do what they feel is right; however, without starting a huge debate, since you asked, I will give you my thoughts on this. I had 3 boys in three years, I know it is so hard to hear your children cry, your instinct as a mother is to comfort them, believe me I know. However what I learned after doing it the wrong way with the first child is, when one doesn't teach their children how to comfort themselves back to sleep on their own, it doesn't do you or them any favors. At 20 months your child should be sleeping through the night. You know he's not hungry or in pain, let him cry it out, contrary to what many people think it is not going to make him think you don't love him or make him feel insecure, I assure you it most likely will not take more than 2 weeks to have him sleeping through the night, quite possibly a lot less. Each time you are going in to comfort him now, you start the process all over again. He knows you are going to come in. When you stop coming in, he will eventually stop the crying, he won't love you any less and feel like you love him any less, but the both of you and your family will be a whole lot happier with two well rested individuals.
Remember when your sleep is interupted you are not at your best as a mother, partner, friend or what have you and your child is not at his best either the following day.
Even on the Nanny show, she is not a follower of the co-sleeping or getting up several times a night with your toddler. I say this, so you know even professionals think this same way.
However to all the co-sleepers and the moms that don't agree with me, out there I'm not condemning what you do, like I said to each their own. You do what you feel most comfortable with, but in this particular situation she does not like getting up several times a night, so this is just my opinion. So this need not turn into a huge debate, b/c this is usually a topic that causes that.