I'm not sure on the specifics, I only know there are 2 paths to PR from a 457. If you take the path that you're talking about you don't need to do the skills test as your 2 years working covers that. With the other path, as far as I know, you can apply for PR at any time as long as your employer is willing to sponsor you.
As a side note, we've now got our visa and should be in Adelaide in June :-)