As I understand it the situation is as follows:
OLD rules (ie.
PR granted 30/6/07 or earlier). Need to do 2 years on
PR for citizenship; any time on temporary visas does not count.
NEW rules (ie.
PR granted 1/7/07 or later). Need to do at least 4 years residence but may count up to 3 years of temporary visa time. That is, you qualify as long as you have *at least 1* year
PR and at least 4 years in total.
Clearly if you do a long spell on a 457 prior to
PR the new rules are 'better' for you
Hope this clarifies