Wendy, it's mostly my preference. I was the same way with DD - just can't bear getting him breakfast that early (he's normally up between 6-6:30, though this morning was a freakish 7:05) and he won't wait more than 10 minutes or so after waking before either bottle or food.
I have to go to Whole Foods tomorrow to buy fish anyway so I might pick up some of those Yo-baby smoothies and offer either that in sippy or yogurt in place of the afternoon bottle. Right now DH is feeding DS his "bedtime" bottle (though he won't go to bed for at least another half hour) and DS is quite happy - but he really did do well without his afternoon one, so at least maybe we can cut down to two bottles and make up the milk elsewhere...
BTW, DH kept his morning bottle until he was almost 6!!!