Text (7th+errata): Whenever you cast a creature spell, you may pay {1}. If you do, return target creature to its owner's hand. [Oracle 2009/07/08]
You can pay for the triggered ability once each time a creature spell is played. [D'Angelo 1998/06/25]
Triggers when the spell is announced, which is before responses to that spell can be announced, and always before the creature it triggers on is put into play. You cannot use Equilibrium on the creature being played. [D'Angelo 1999/06/01]
A creature spell is any "Creature" or "Artifact Creature" spell. [D'Angelo 1996/06/01] Older cards of type Summon are also creature spells.
