@alk
Thank you for your posts.
I've tried both ways and got it working. The first way (pc internet access over iphone connected via usb) works fine, good for using iphone as a modem (which is not allowed by my provider) even without wifi.

The other way (iphone gets internet access via local proxy) works too, but i dont understand why my email accounts still work (I DID NOT configure smtp and pop3 ports). I edited the proxy config to log the traffic and the connect to the mail servers is not logged only the traffic when i open safari on the iphone.
For me this means that port 80 traffic goes to the proxy and everything else (maps? youtube?) still goes to the edge/gprs provider.
This is not good. Its hard to see when things go wrong and if you have expensive data plans you might think that everythings ok but in fact you'll be charged.
I disabled egde with bossprefs but after that (as expected) i have no connection at all.
Is there a way to be sure that the local proxy is the only internet connection for the iphone?
btw I'm on 2.0.1 pwned, officially unlocked and activated.

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8GB IPhone
Victim of T-Mobile Germany