It's templated much like every other 5th ed comes-into-play ability. The 6th ed rules change is what caused the errata, not Pandemonium, AFAIK.
Wrong. Read [card]Nekrataal[/card] or [card]Uktabi Orangutan[/card]. They're templated the exact same way, but they don't have complicated errata. The 6th ed. rules left those guys alone, but Dreadnaught has been trashed.
Under 5th ed. rules, "When this comes into play" abilities resolved immediately after the creature resolved, with no opportunity to respond. Dreadnaught still works like this, but the 6th ed. rules changed how Uktabi & Co. worked in a big way, by letting you bounce or Shelter or whatever
in response. This has become so important to so many cards that people forget that it didn't always work this way. Previously, you had to choose which creature to save
before the Nekrataal player decided what to kill (i.e. during the fast effects window before the Nekrataal resolved, after interrupts and damage prevention. Pre-sixth rules
sucked).
Dreadnaught had its original wording intact at least into 1999, where the PandeNaught combo was the kill in the first Necropotence-fueled combo deck (invented by Adrian Sullivan, called Dread Panda Roberts). It was pretty easy to play Naught and deal 12, then Animate Dead or Reanimate for 12 more.