So there are probably plenty of people that want to switch, but there's no way to do so without completely rerolling. Yet, anyway. Here's the best idea I've heard on the subject so far: create a quest that would let players change factions.
Yes, I know, there's lots of reasons for Blizzard not to do it (not least of which is the whole interfaction communication thing). But consider it seriously-- it would be a once-in-a-lifetime quest that would have serious costs (so no nightly switching back and forth), and take a long, long (maybe even a series of daily quests) time to get done. This isn't some frivolous thing-- it's a serious decision, a second chance to reconsider a faction choice you might have made 70 levels ago. Right now, you only get once chance to choose a faction, and it's before you even know your character. Surely you should get one chance to reconsider, right?
And originally I was thinking that a faction change would be a "Horde in Durnholde" kind of thing-- if you're an Orc, then you suddenly become Human forever. But Blizzard could even work it so that you could stay the same race, and just switch sides. Imagine it-- Gnome and Undead Rogues alongside each other, dogs and cats living together, mass hysteria! Although it would get confusing in the battlegrounds, wouldn't it?