Prices start at $49 a month for the 16GB including 4GB of included data allowance on Optus' 3G network, and go up to $59 a month for the 32GB version with 8GB of data. The 64GB version is also available for $10 a month on the $59 plan, with all plans lasting 24 months.
Business customers also have the added benefit of being able to bundle an iPhone 4S and share data between the two devices. This actually sounds like a really good idea - chances are you won't use all your bundled data anyway, so sharing it between two devices is a great way to save costs. Hopefully this will make its way to the consumer option as well.
Ultimately it's going to cost more than buying the iPad outright, but if you know you want to use Optus as your 3G provider of choice and don't mind paying the premium, it could be a viable option.