2006-09-25 : A face-down creature spell will cause Opal Guardian’s ability to trigger (if it’s an enchantment).

2006-09-25 : After the triggered ability resolves, Opal Guardian is no longer an enchantment.

2006-09-25 : Guardian’s effect doesn’t wear off.

2008-08-01 : A noncreature permanent that turns into a creature can attack, and its {T} abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn’t matter how long the permanent has been a creature.