Some acidic foods and drinks, especially sodas, temporary make the outer most layer of enamel weaker for about 30 minutes. Brushing during that time can brake off pieces of this weakened enamel. After 30 minutes, our enamel is back to its previous state in most people and can safely be brushed.

see the complete at
http//www.http://well.blogs.nytimes.com/2012/05/21/really-never-brush-your-teeth-immediately-after-a-meal/