Well, its false in that you can't say you "physically can not get pregnant" because people have plenty of times.
Its less likely to get pregnant. How much less likely? It really depends on the person in question. It also has to do with how much your nursing. Is the person nursing exclusively, or is the baby having solid foods or supplimenting?

After her first child, every single one of my best friend's children were concieved while she was breastfeeding one, or even two of the other children.
I'm sure if you went to a La Leche meeting, you could easily find more then a few people who also had their second child while nursing the first.