Aeuis brings up an interesting point over on the forums: even though some of the items we get are called "Bind on Account" (or, more properly, Bind to Account, even though most people use BoA as shorthand) they're really not that, because you can't actually send them across factions or realms. Of course, that's pretty obvious to people who've been playing the game for a while -- Blizzard has never really encouraged cross-faction interaction at all (you can "send" items through the neutral AH, though even that is a bit of a hack, not really the intended purpose). So BoA items are actually bound to account, server, and faction.
But Wyrxian is surprisingly open about "future possibility:" he does say that while Blizzard could allow us to send items between factions if they wanted us to, they also wanted to leave things open just in case the functionality came into view in the future. That doesn't mean we can expect a change in the next patch, but it does seem to mean that Blizzard is still open to the "possibility." You'd think it be tough technically (character inventories are all over the place, especially cross-server), but considering that we can now have cross-server battlegrounds, all it would probably take is a tweak to the in-game mail system. One character sends an item, another character receives it.
And really, the only possibility that might come open is just ease-of-use -- that's why BoA and Heirloom items are in the game already, and if Blizzard determines that it would be easier to have you share items between your characters' factions and servers (perhaps if they upgrade the character re-customization feature to include a faction change?), then it will likely happen sooner or later.