Sanya says WAR!
Sanya (Mythic Community Relations Director) sets everything straight when it comes to what acronym to use when talking about Warhammer Online. Here's her direct quote from the WARHerald:
I've been seeing some people use the acronyms WHO and WAR interchangably. When I delicately offer correction, I get a shrug and a "potayto potahto" reply.
First of all, it's totally pronounced "POTAYTO."
Second: WHO does not belong to us as a trademark and wasn't approved by GW, so I won't use it.
Third: Warhammer is one word, anyway!
Fourth: WAR is much cooler. Would you rather tell your mate that you're going to go play WHO for two hours and waste precious game time getting into a terrible Abbott and Costello routine, or say you're going to play WAR for two hours?