Text (CH+errata): Switch target creature's power and toughness until end of turn. [Oracle 2005/08/01]
As per Rule 613, power and toughness switching is applied after any effects that modify power/toughness. This means that even if new spells and abilities are played to modify power/toughness, they are applied before the switch. [D'Angelo 2005/12/01]
This can effectively kill a creature with a power of zero, but there is still a chance to increase the power of the creature using instants before this takes effect. [bethmo 1994/06/14]
Any additional words on cards which modify the power/toughness are also reversed. For example, Blood Lust acts as if it read "Target creature gets -4/+4 until end of turn. If this reduces that creature's power to less than 1, the creature's power is 1." [WotC Rules Team 1997/07/03]
Extended tournaments (see Rule 913) have banned this card since 1999/10/01.
