Our ped recommended apple or white grape juice at 2 months, which I (and everyone else I know) thought was much too young. I started my baby on juice around 4 months.
As other posters have noted - no more than 4 oz a day, diluted 50% with water. Don't bother with the gerber or other bottled 'baby juices' - they are just diluted juice (i.e. you are paying for the extra water.) I buy organic juice and add filtered tap water.
Our baby is now 1 year old - will drink juice (still diluted) water and milk with equal gusto. We still haven't given her orange juice, but apple, grape, cranberry, peach, etc. she enjoys very much.
One more quick thought - you should also ask your dr. how much water to give your baby. Because they're so small, babies can get over-hydrated very easily (water poisoning - if you can believe it.) In fact, some books I've seen recommend not giving water OR juice.