I think alot of Narnia fans are going a little overboard with this.
We ALL know what the books say. Unicorns should only be ridden in times of dire need, and that talking beasts should not be ridden except in times of need.
Let's see, a war against the greatest evil that Narnia had ever saw(and quite possibly the greatest ever) seems like a DIRE emergency to me. And like someone else pointed out, the unicorn may have wanted Peter, the High king, to ride him/her. Talking beasts pass on stories to their grandchildren(or grandfoals) too. I can hear the unicorn now "When I was your age, I allowed Peter the High King to ride me in the great war against the White Witch and the Hundred Years' Winter." It doesn't really bother me, because it was about the same situation with Tirian and Jewel(IMO).
As for Edmund and Phillip. Once again, when Edmund is practicing his swordmanship, it is kinda an emergency. Who better to tell Edmund how to fight on a horse than a horse? The Hunt for the White Stag, bothers me a fare bit, but not too bad. I mean that was definitely a "special" occasion, and didn't the Hunt for the White Stag only come around every so often years(I don't remember quite clearly now)? So I guess that doesn't bother me so much. Special occasion calls for a special horse.
Those are my thoughts.