I, too, work in a peds office and see some (but not all) of our breastfeeding moms and their babies.  (I *wish* I could see them all - seeing only the problem that no one else can handle gets depressing...)  
I've had the same experience as Karin many times with mothers deciding that it is unfair to the older child to "take the time" to breastfeed the new baby.  Or they say that the older child is "jealous" of the new sibling at the breast, but not of bottle feedings.  I've tried the same approaches that Karin mentions, and generally without success.
I've mulled this over a lot, and it seems to me that the women who make this choice must not have experienced a truly pleasurable breastfeeding relationship with the older child.  It must have been "OK", but not a delight, or they would never consider bottle feeding the new baby.  Chances are, we would find that these women supplemented early, and weaned to bottles early.  Certainly their level of comfort with breastfeeding doesn't allow them to offer the "jealous" older child a try...
My guess is that the only solution to this is better support and education the *first* time around.  Parenting styles vary among breastfeeders, obviously, and it is not likely that we see this "I can't nurse this baby because it isn't fair to my first" thing in attachment-style parents.  Attachment-style moms have bigger laps, longer arms...  IMHO.
Willow in RAINY Detroit, where winter is on vacation.