Apple has been inviting journalists to a press conference on its Cupertino, Calif., campus at 10 a.m. Pacific time Friday, the subject being the iPhone 4. But what about the iPhone 4 would require the company to summon the press on such short notice? Could Apple be introducing a new case for its smartphone? Will the company make it in colors besides black and white? Is Apple on the verge of finally granting some techies’ fondest wish by introducing a push-to-talk iPhone for Nextel?