Recently Apple released Version 3.1.3 of the iPhone OS, reenabling tethering for Carriers which do not originally have the iPhone. I myself got an iPhone from italy completely unlocked running on the O2 Network in Germany, until Version 3.1.3 I was using a costum IPCC Carrier Bundle file to get a nice O2 logo and also to have my APNs and MMS preconfigured so I didn't have to do that everytime I change SIMs. Sadly this IPCC stopped working with 3.1.3, well it did not stop working technically but tethering was not enabled while using it, also the MMS settings didn't get applied properly.

Googling around for some time last night I was able to setup a carrier bundle for O2 which enables tethering, pre populates all the APN and MMS settings, while getting rid of those nasty "Call forwarding" messages when calling somebody, so for anybody interested you can download it O2 Germany Carrier Bundle

To apply it simple run

defaults write com.apple.iTunes carrier-testing -bool TRUE

in the Terminal with iTunes closed to enable custom carrier bundles. Afterwards you can apply the IPCC simply by Option+Click on Check for Updates in iTunes and selecting the downloaded IPCC.