Based on other threads around the forum: If you are planning to use the iPhone 3G in the USA with other carrier different from AT&T, assuming you "can" unlock it, the data connection via cellular network will not work in 3G, you'll be limited to EDGE.
Hopefully 2 things will happen sometime in the future:
- the good iPhone Developers community will find a way to unlock the iPhone 3G, the most bold ones will try to sell it via the App Store

, and will find a workaround to use the App Store for enterprise and we could have a similar Installer/Cydia type of distribution channel.
- After the initial months of the iPhone 3G boom, the carriers, specially in Europe or even Apple may sell them without the carrier subsidized price, so you can buy paying it's "real market value".
And of course you will always have the alternative to buy from eBay, Craigslist, etc...
Just wait and see what happens...
Another alternative is to find a job that requires you to be always available and the company will assign you an iPhone 3G, so you don't have to worry about the contract....

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Procar