On the iPhone 3G, and WITHOUT jailbreaking.
In iTunes, it tells me "your account hasn't been set up for iTunes", and takes me to a form where I have to fill in my credit card info.

Have u ever a used a hacked one?
I mean seriously people u dont have to pay, just need some cracked apps

Yes.

The credit card is used for id purposes in addition to the credit for purchases. If you make free purchases it will not charge your credit card, it just ties that information with you for updating and theft tracking purposes.

good luck.