Optus have sent around an email to current iPhone 3G customers offering up to 50% off the upgrade fee when you sign up for the new 3GS handset for 24 months.
The percentage that you actually get seems to very between 20% and 50%, and looks to depend on how far into your current contract you currently are.
The actual upgrade fee itself for each person also varies, and apparently won't be made known until tomorrow (although some people are saying that Optus are telling them when they call up). However, if you are still paying off your current 3G handset, you'll still have to pay that off on the new contract as well – so make sure you factor in that continued cost when if you decide to upgrade.
The email is also requiring potential iPhone 3GS owners to register their place in the queue for tonight's midnight launch, so if you were planning on just rocking up tonight, you might want to look into trying to register.