iPhone 3.1.3 has just recently been released. Rumours of tethering working with officially carrier unlocked iPhones with unofficial sims…..
Yes Officially unlocked iPhones (such as sold in New Zealand) will work with any other unofficial carriers SIM and Tethering Works
No released jailbreak as of yet for the 3GS
to access this service go to http://www.unlockit.co.nz from your Iphone