[8bitpony]

My dentist has advised me not to brush after a sugary meal or drink for at least an hour, otherwise you are effectively brushing the sugar into your teeth. Perhaps this also applies (but with a lesser timeframe) for meals that aren't so sugary.

[nilkn]

It's more that your enamel is weakened temporarily by acids (which get produced by bacteria in your mouth as a side effect of their consuming the sugar). It is true that you shouldn't brush immediately after a meal -- waiting 30 minutes to an hour is a good practice.