essientally this is the same hack as the 3.0 but minor changes. im currently working with @terraboss (twitter) on making this work on unsupported carriers. Use the 3.0 and post your results but the ipcc i post wont work on unsupported carriers. it could actually make it worse.
POST your configuration:
iphone
country
carriers

Unfortunately it didn't work

My iPhone is 3GS running 3.1.2, I'm from Kuwait and my carrier is Zain KW (41902)

I can confirm T-Mobile Germany with your ipcc File does work. But I do not know why, because your file has wrong carrier settings and original T-Mobile Germany bundle has additional apn only for tethering (which should only work with tethering option contract, which I do not have). I did not use additional ipcc or settings file...

writing this tethered by bluetooth and original T-Mobile Complete S (1st Gen) contract.

I am still on (pwned) iPhone 3G 3.1 (7C144), Baseband 05.11.07, factory unlocked iPhone 3G from Czech republic, Carrier T-Mobile 5.0, original iPhone contract without tethering option, iTunes 9.01, mywi 3.2.4, my mcc/mnc = 26201

__________________
Every time you Can Has, God kills a LOLcat.

too complicate!!

try this and it works perfect!
HowTo: Tethering FW 3.1.2 (Free) | Apfelphone.net - ?cause it?s not just a phone

so couldnt i just use you ipcc file for ATT and not jailbreak?

Nice step by step how to.

Original credits?

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JSn1™

Guys, no need for any doodle carrier bundles. Just use any custom made like from here
Carrier Bundle Generator iPhone OS 3.0 - 3.1.2 (ipcc)
or your own and install MyFi. Thats all. Besides other options MyFi enables the tethering switch and function, MyFi does the job, not the modified bundle..
Working on my 3GS 3.1.2, my 3G 3.1.2

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Read the stickies and search the forum before posting!
If you want to become a Hackint0sh supporter click here
----------
iPhone 4 factory unlocked, iOS 4.0.1, jailbroken
iPhone 3GS factory unlocked (3.1.2; Blackra1n, Cydia, OpenSSH, custom 3.0 ipcc file (no sig), tethering hack by Dev team, 32 GB)
iPhone 3G (3.0.1; Redsn0w 0.8, Cydia, OpenSSH) 16 GB

Sorry my friend... but your way is too circuitous...;oP

That's my solution:
How to Enable Tethering on iPhone OS 3.1.2 | iPhone Guides (Step-by-Step) | iPhoneHeat

Enjoy it! No ipcc hacks..
...no stress with extra costs

Thats what my post was saying. Custom carrier bundles are always the way to go if you are not on an Apple supported carrier. Just saying that you can use any custom carrier bundle, 3.0's are working fine.

__________________
Read the stickies and search the forum before posting!
If you want to become a Hackint0sh supporter click here
----------
iPhone 4 factory unlocked, iOS 4.0.1, jailbroken
iPhone 3GS factory unlocked (3.1.2; Blackra1n, Cydia, OpenSSH, custom 3.0 ipcc file (no sig), tethering hack by Dev team, 32 GB)
iPhone 3G (3.0.1; Redsn0w 0.8, Cydia, OpenSSH) 16 GB

if you use the version of apt.apfelphone.net then you have a full version of the program without any trial time!