While I am not with Vodafone Australia but Optus, and I used to have an iPhone 3G (gave it away), I was brought to the attention of a Macworld article that announced tethering will be free for Vodafone customers, and you would not have to add on another data plan, for which Optus customers would have to fork out.
Vodafone will offer the iPhone 3G S 16GB model for free on its $69 per month plan over 24 months, including $400 of calls and 1GB of data, and the 32GB model for free on the $114 per month plan with unlimited calls and 2GB of data.
So basically, users would be able to use their existing data quota on tethering, which is how it should be for any of the other carriers. You already pay for the quota, you should be able to do with it whatever you want. Any of you overseas, any news on the plans your carriers will release with the upcoming iPhone?