25 Sept 2006
A face-down creature spell will cause Opal Guardian's ability to trigger (if it's an enchantment).
25 Sept 2006
After the triggered ability resolves, Opal Guardian is no longer an enchantment.
25 Sept 2006
Guardian's effect doesn't wear off.
1 Aug 2008
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.