We have the new login screen that might be the final version for Mists of Pandaria. All the beta states of WoW were showing their current expansion screen, the current beta is showing two pandaren statues, similar to the ones from Lord of the Rings (The Fellowship of the ring), with a combo music from Mists of Pandaria, old Vanilla and The Burning Crusade.
If we look in the past, all login screen have shared the vanilla epic intro music, but all having different styles.
Mists of Pandaria login screen and music.
WoW Vanilla. Having a portal, the one like we seen in some missions from Warcraft 3.
Burning Crusade login screen, a modified version of the one from Vanilla, most of the fanbase consider this to be the best WoW login screen. The portal is the one from Badlands WoW zone that is leading to Outland, the place where all the players started the expansion.
Wrath of the Lich King login screen, having Icecrown, remembering of the second and last Warcraft 3 addon, “The Frozen Throne”. Blizzard has reached the peak in number of subscribers with Wrath of the Lich King, this is the expansion that was the start for most of World of Warcraft players.
Cataclysm. The current expansion, having Deathwing above Stormwind.
There is no official way of changing the login screen, avoid any software that is claiming that can change the login screen as most of them are intended to steal password and hack accounts.