You can buy the iPhone 4G without being a current AT&T customer (I thought this was what you were asking). It seems that what you were really asking is if you need to sign up for AT&T service when you purchase the iPhone, which I'm pretty sure the answer is yes. If you're willing to pay a crapload of cash for it, you can get the iPhone 4 without the 2 year contract requirement, but you'll still need to sign up for AT&T service.
Don't be so confident about an unlock for the iPhone 4. Sure, the people that find the holes in the baseband are really really good at what they do, but it could potentially take them months to finally find an exploitable hole. No one will know for sure if an unlock is possible until the iPhone 4 is released.
They already have a hole