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The "Tether" app in Cydia only works for 3.0 and 3.0.1 as of now. 3.1 users will have to wait until 10/10/09 to tether on jailbroken 3.1. For those who don't want to wait it out, here's a step by step tutorial to downgrade to 3.0: http://www.benm.at/tutorials/howto-iphone-3g-firmware-3-1-downgrade-os-x/

Note: This will only work on Mac OS X and iPhone 2G/3G, NOT 3GS.

Or you can use PDAnet or Miwi (I think). All discussed here. https://forums.macrumors.com/threads/780940/
 
I took a look at those. They only work if you have a wifi connection, which pretty much defeats the purpose of having to tether!

No. They wifi tether... That means your laptop needs a wifi card. That's it.

You create an Ad-Hoc network on your laptop, connect your iPhone to it and run the app. Your Laptop will then pull internet from the phone.
 
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