I believe that Apple requires payment methods for all app downloads, even the free ones. One reason might be to assist in the age verification issue for apps. There are apps I get wornings for about age related content every time I download an update for one of those apps.
this means you have a out standing balance with itunes. contact itunes by going to expresslane.apple.com and then ask him what you owe or go to itunes on a computer> account> and look at past purchases> that will show you what it owes