My dentist says the opposite, since toothbrush bristles don't get between the teeth. According to her, brushing after flossing just pushes all the dislodged stuff back between the teeth.

In my opinion, it doesn't really matter which you do first, as long as you rinse well after each step. I brush first - get most of it with the brush and rinse, then get the rest of it with the floss and rinse.