Optus are holding an event right now unveiling their iPhone 3GS pricing. The jewel in the crown: a $99 'Yes' Timeless Plan with unlimited standard national calls and text, 1.5GB of mobile internet browsing, tethering included. It's slightly more complicated than just that $99 monthly fee over 24 months. It's only available for customers signing up to the $99 iPhone 3GS plan who take the $15 monthly Mobile Internet pack, although that $15 a month fee is credited back each month. After your 24 month contract is up, you'll start paying the $15 again though.
Tethering is included in the Timeless plans above, but will be the $10 monthly fee for all the other plans on offer. One of the reasons for this is to ensure network quality apparently. Also worth noting is the fact that the only reason (according to Optus) the iPhone has a tethering charge while other phones don't is that the iPhone is the only device with data included in the price.
This pricing structure is a limited time promotion for about three months or so. Minimum price over 24 months is $2,376 on the $99 Timeless plan. There's a heap of other plans available, as you can see below: As you can see, the only way you can get a new 3GS for free is via the Timeless plans above. You can pick up an 8GB iPhone 3G on a $59 cap plan though, which isn't too bad an option I guess.
For what's included with each cap, you can head over to Optus' website, although the $99 Timeless options looks to be the way to go if you're looking at Optus for the new iPhone.
UPDATE: A few of you have asked about pre-paid pricing. Short answer is that it doesn't exist. Longer answer is that it's something Optus will be looking at, but not for launch... [Optus]