From what I heard it was supposed to be the first Source game coming to the consoles. So that means a set release date was before the Orange Box's release but I guess something happened and it got delayed. It sucks, but what I'm concerned about is they are going to use a 3rd person perspective instead of a first person perspective which kinda worries me. I love the game being in first person rather than 3rd person.
The Source engine might make Postal better. It's an old engine but you can still get a lot out of it if you take your time. Hopefully it will be good. And maybe you'll be able to use the many props, ragdolls, etc into Garry's Mod.
It uses the Source Engine not the Orange Box Engine since there isn't one.
The Orange Box engine is a modified version of the Source Engine. It's still Source but it's modified to support dynamic shadowing and lighting as well as dual core support.