Apple Inc. can't lay claim to be the world's most valuable company — for the next day, at least. The Cupertino-based computer and mobile device maker slipped to No. 2 on the list Thursday while Microsoft Inc. took the top spot. The Redmond, Wash.-based computer and software maker, was worth about $7 billion more than Apple by the end of the trading day. The shift was just one of the changes to the rarified world of companies we're calling titans.