iPhone 4 internet tethering stopped

MacBook Pro 2.8 GHz OS 10.6.8

iPhone 4

Yesterday the internet tethering suddenly stopped.

I think it happened after software update which installed the latest version of iTunes.

Otherwise the iPhone works normally; I have 3G connection, can browse and what not.

iTunes works and communicates with the iPhone normally.

But no internet tethering. I have

* checked and replaced the USB cable.. But no internet tethering. What could this be?

* reset the network settings on the iPhone

* called the operator

* stopped the firewall in System Preferences

I'm clueless.

Greetings

Donald