11/1/2005: Note that the wording "Effects that alter the creature's power alter its toughness instead, and vice versa, this turn" has been removed.
10/1/2008: Effects that switch power and toughness are always applied last when determining a creature's power and toughness, even if other power/toughness-changing effects are created later in the turn. For example, if you activate Phantasmal Fiend's first ability, it will become 2/4. Then if you activate its second ability, it will become 4/2. Then if you activate its first ability again, it will become 3/3.