2013-04-15 : Any enters-the-battlefield abilities of the copied creature will trigger when Progenitor Mimic enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.

2013-04-15 : If Progenitor Mimic somehow enters the battlefield at the same time as another creature, it can't become a copy of that creature. You may choose only a creature that's already on the battlefield.

2013-04-15 : If the chosen creature has {X} in its mana cost (such as Protean Hydra), X is considered to be zero.

2013-04-15 : If the chosen creature is a token, Progenitor Mimic copies the original characteristics of that token as stated by the effect that put it onto the battlefield. Copying a token doesn't make Progenitor Mimic become a token.

2013-04-15 : If you choose to have Progenitor Mimic enter the battlefield as a copy of a creature, the triggered ability it gains will become part of its copiable values. For example, suppose Progenitor Mimic enters the battlefield as a copy of Runeclaw Bear, a 2/2 green Bear creature with mana cost {1}{G}. The resulting object is a 2/2 green Bear creature named Runeclaw Bear with mana cost {1}{G} and with “At the beginning of your upkeep, if this creature isn't a token, put a token onto the battlefield that's a copy of this creature.” If another Progenitor Mimic enters the battlefield as a copy of that creature, it will be a Runeclaw Bear with two instances of the triggered ability.

2013-04-15 : Progenitor Mimic copies exactly what was printed on the original creature and nothing more (unless that creature is copying something else or is a token). It doesn't copy whether that creature is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.

2013-04-15 : The original Progenitor Mimic will create tokens, but those token copies will not.

2013-04-15 : You can choose not to copy anything. In that case, Progenitor Mimic enters the battlefield as a 0/0 Shapeshifter creature and is probably put into the graveyard immediately.