According to the client's ServerMessages.dbc (which should contain every piece of text the server can automatically announce), Mists of Pandaria may be different:
The Cataclysm message is still there, unchanged. It still refers to Cataclysm: "Cataclysm content is now available. Please completely quit and restart World of Warcraft, then enjoy the game."
In contrast there are 3 new Mists of Pandaria messages, which simply state:
- "Mists of Pandaria has launched!"
- "Mists of Pandaria has launched! Visit Orgrimmar to begin your adventure!"
- "Mists of Pandaria has launched! Visit Stormwind to begin your adventure!"
Gut feeling is that the first is for the benefit of those without an upgrade, and the other 2 are for Horde and Alliance who have upgraded. Critically the MoP message make no reference to restarting, as in Cataclysm.
That makes sense, because Cataclysm was a fundamentally more complex expansion, with new zones in existing continents. In contrast access to Pandaria can be turned on by enabling a few quests, something that does not require a client restart.