2004-10-04 : Phasing in does not trigger “enters the battlefield” abilities, so you don’t have to sacrifice again if it phases in.

2004-10-04 : The sacrifices are simultaneous, not one at a time.

2007-07-15 : If the Dreadnought is no longer on the battlefield when the triggered ability resolves, you may still choose either option.

2007-07-15 : Reverted to its original wording, this now has an enters-the-battlefield triggered ability. During resolution of the triggered ability, you choose one option or the other.

2013-04-15 : If you are unable to sacrifice 12 power worth of creatures, then you cannot choose that option; in that case, you must sacrifice Phyrexian Dreadnought.