Expanding on [Logicbird's answer][1], here is what the [British Dental Health Foundation][2] says:

> **Acidic foods and drinks, such as
> oranges, grapefruit and fruit juices**
> that are often eaten at breakfast
> time, **soften the enamel on your teeth**.
> **Brushing immediately afterward wears
> the enamel away**, and can cause dental
> erosion, which may lead to pain and
> extreme sensitivity in the teeth, and
> also lead to cosmetic problems.
> **The saliva in your mouth neutralises
> the acidity** and restores its natural
> balance. However research has shown
> that **this can take up to an hour**.


Their tips for avoiding dental erosion are:


> - brush teeth before breakfast if you    have fruit or fruit juice, or 
> - wait one hour after eating or    drinking anything acidic before   
> brushing 
> - use a straw when drinking acidic    drinks to reduce contact with teeth 
> - drink water and milk between meals in    preference to juice and fizzy
> drinks 
> - chew sugar-free gum - this will    produce more saliva to help cancel   
> out acid in your mouth 
> - finish a meal with cheese or milk to    help neutralise any acids

The [NHS][3] says the same:

> Brush your teeth in the morning before
> breakfast and last thing at night
> before you go to bed (and ideally at
> least an hour after your evening
> meal). 
> **Brushing your teeth straight after a
> meal can damage your teeth, especially
> if you've had fruit, fizzy drinks,
> wine or any other food that contains
> acid**. This is because tooth enamel is
> softened by the acid and can be worn
> away by brushing. Waiting an hour
> gives your saliva chance to neutralise
> the acid.

  [1]: http://skeptics.stackexchange.com/questions/2411/whats-the-point-of-brushing-teeth-in-the-morning/2413#2413
  [2]: http://www.emaxhealth.com/79/1063.html
  [3]: http://www.nhs.uk/Livewell/dentalhealth/Pages/Teethcleaningguide.aspx