Ballmer really seems to have no taste. Taste is a matter of taste, of course, but his nasty tone toward Apple comes off as very immature. To be antagonistic toward Apple is natural; it is the way he expresses that antagonism. Flippancy and arrogance never look good. (I am thinking of other things he has said, for example early remarks about the iPhone after it was announced.) When Jobs makes a remark about Microsoft in public, it is, as far as I have seen, a much more friendly type of joking, as opposed to Ballmer's hateful tone.
In this particular case, there is not enough information in that article to tell. It is quite reasonable to joke with the employee for using an iPhone. It's a bit of a slap-in-the-face gesture to present an iPhone at a Microsoft event, and it probably just didn't occur to the employee; Ballmer, realizing the employee didn't realize that, was in a position to make fun--friendly fun. Like when you say something to a friend which, literally taken and out of context, sounds like an insult, and the friend is mock-offended. Given Ballmer's insane (though, to me, tasteless and excruciating) style--all that jumping and whooping--pretending to trample the iPhone fits right in. It would all depend on tone, gestures, and facial expressions.
What kind of man Ballmer is and Jobs is, none of us know for sure; I restrict my remarks entirely to politics and public image.