Tethering is free (and currently available, I tried it out) on Optus in Australia. I don't know the situation with the two other Aus iphone carriers.
The SMH is reporting that it'll be $10/m from 22nd June (http://www.smh.com.au/digital-life/mobiles/optus-iphone-teth...), and the Optus FAQ says it will announce the price on that date (http://personal.optus.com.au/web/ocaportal.portal?_nfpb=true...).