Possibly, but that's because Optus has a pretty sucky network for everyone. I understand (from reading whirlpool) that their new 3G network is not iPhone compatible as it's 900MHz
I think your information's out of date. I use Optus 3G on an iPhone in an area which was in exactly that position, but they upgraded a week before the iPhone came out. I think that upgrade happened nationwide. (Not that it's relevant to this thread, I know, but...)